ABSORPTION "I- l.ltiMT 



ABSTINK' 



The mod* of absorption of light by a coloured substance is often 

 eminently characteristic of the substance, but can be judged of only 

 very imperfectly *7 the tint of the transmitted light It u easily 



fterrained by analysing the transmitted light by means of a prism. 

 When a body i not nomogenvoiw in structure,, but (like chalk or 

 paper) u filled with surface* capable of reflecting light, a considerable 

 portion of the whole reflected light ordinarily proceeds from a greater 

 ur ! depth beneath the outer surface. If the material of which the 

 body U composed be one capable of absorbing light, the light reflect.*! 

 from the interior suffers absorption both in penetrating into the body 

 aad in getting out again. Accordingly, those coloui - hicli the nwte- 

 rial i linsl disposed to abaorb are found predominating in the reflected 

 light. It i* thus that abaorption operates in the case of pigment-, tin- 

 pvtali of flnwem, tlyed clothe*, Ac., which exhibit more or leas lively 

 colours by reflected light, though in these canes the light reflected 

 strictly at the outer surface is colourless. The tint of the reflected 

 light has a general agreement with that of the light transmitted through 

 sufficiently thin stratum of the coloured material 

 Metals may for most purposes be regarded as absolutely op:u|iu- : \ > 

 even they can rtoiuetimee be rendered m> thin an to transmit light. 

 Thus, gold-leaf transmit* a green or bluish-green light ; and the 

 coloration of the light shows that the transmission does not take 

 place merely through the minute hole* with which gold-leaf is filled, 

 but actually through the metal. There are good reasons for believing 

 that the strong reflecting power of metals U intimately connected with 

 their intense absorbing power. Thus gold, which absorbs the more 

 refrangible colours with most energy, reflects them also in greatest 

 abundance, so that it is yellow by reflected, while it is bluish-green by 

 transmitted light. Some intensely coloured substances (murexidc and 

 platinocyanide of magnesium are good examples) absorb the colours of 

 a part of the spectrum almost as intensely as metals, while for other 

 parts they are comparatively transparent, and reflect the colours for 

 which they have an intense absorbing power with an. energy comparable 

 with metals : while other colours are reflected only as they would be 

 by vitreous substances. Hence the rtyularli/ reflected light is brilliantly 

 coloured ; but the predominant colour is that of the light most intensely 

 absorbed. This is just the reverse of what takes place in the case of 

 the light reflected from the petals of flowers, &c., where the coloration 

 U due, not to reflection, but to absorption, and the colour is that due 

 to the light for which the absorbing power of the colouring substance 

 U Ifatl. (See a paper by M. Haidinger in the Proceedings of the 

 Academy at Vienna. ' Sitzungsberichte,' Bd. 8, S. 97. See also ' Phil. 

 .Mag.,' & 4, vol. vL, pp. 284 and 393.) 



When a doubly refracting crystal is coloured, it often happens that 

 the two pencils which, in any given direction within the crystal, are 

 capable of being transmitted independently of each other, are very 

 differently absorbed. Thus a plate of tourmaline cut parallel to the 

 axis stops more or less completely light polarised parallel to the axis, 

 constituting the ordinary ray, and lets through light polarised perpcn- 

 dicularly to the axis, constituting the extraordinary ray. In some 

 specimens, with plates of a suitable thickness, the Htop]>age and 

 transmission respectively are tolerably perfect, which makes such a 

 plate very valuable in experiments on polarisation. That the effect is 

 really one of absorption may be shown by using, instead of a plate 

 bounded by parallel surfaces, a thin wedge tapering to a mere line, and 

 viewing it separately by light polarised parallel and perpendicularly to 

 the axis. It U found that quite close to the edge the crystal is 

 colourless and transparent for both pencils ; but that as the distance 

 from the edge increases, the ordinary pencil becomes rapidly more and 

 more absorbed, while the absorption of the extraordinary comes on but 

 lowly. As usual in absorption, the different colours are unequally 

 absorbed ; and not only so, but the colours which are most absorbed 

 are different for the ordinary and extraordinary rays, so that the crystal 

 is commonly differently coloured with regard to the two pencils, which 

 may be observed at the same moment, but separately, by viewing the 

 crystal through a double-image prism. Tin- mode of absorption 

 < hange, not only in a given direction within the crystal in pawing 

 from the ordinary to the extraordinary ray, but also in passing from 

 one direction to another. Dr. WoUaston appears to have first 

 observed (' Phil. Trans.,' 1804, p. 428) that the light transmitted along 

 the axis of a crystal of tourmaline had a colour different from tli.it 

 of the light transmitted perpendicularly to the axis. Several uniaxal 

 i rystals (such as smoky quartz, Ac.) agree with tourmaline in the 

 general character of the absorption which takes place in them. The 

 colour, and generally the mode of absorption, of the ordinary ray is 

 alike in all directions ; that of the extraordinary varies from that of the 

 ordinary, which it has in the direction of the axis, to that most 

 different from the ordinary, which it nan in any direction perpendicular 

 to the axis. Many biaxal crystals have a similar property, but the 

 variation of the colour with the direction is more complicated, and in 

 (Articular some very curious appearance* are observed about the optic 

 axes. (See a paper by Sir David Brewster, ' Phil. Trans.,' 1819, p. 11.) 

 M. Haidinger has shown that in biaxal crystals (or at least in those 

 which are symmetrical with respect to three rectangular planes) there 

 are Circe fundamental modes of absorption, symmetrically related to 

 the principal axes, sera each in any direction perpendicular to the axis 

 10 question by light polarised perpendicular to that axis. 



>4 d 3Mmiii"iit IMI n'cently nhown ('Auiialen de Chiinie. 1 S. ".. 



torn. xli. p. 319) that the power of double absorption may be conferred 

 on naturally colourless crystals by a small amount of foreign impurity. 

 A very remarkable example is afforded by nitrate of strontia coloured 

 red by crystallising out of an infusion of logwood. 



ABSTINENCE, from abttituo, to abstain. The term abstinence 

 signifies a total, or an excessive privation of food. The constituent 

 matter of the body is in a state of continual change the old particle* 

 are constantly taken up and carried out of the system, while new 

 particles are as regularly deposited in their room to repair the Iocs. 

 The source of these new particles is the aliment or food ; but a second 

 office is performed by the aliment scarcely less important than Out < 

 furnishing new matter for the renovation of the system. All the 

 organs of the body are excited to the performance of thrir fun 

 by certain external agent*, which are called stimulants ; such as air, 

 water, heat, and so on; but of these stimulanU the aliim 

 the most indispensable and the most powerful. I'pon tin- quantity 

 and quality of the aliment depend the quantity anil M n..lity 

 blood, and upon the quantity and quality of the blood depends in a 

 great measure the energy of all the functions of all the organs. -\n\ 

 material cliange in the diet must necessarily produce a powerful im- 

 pression on the system. Life can be maintained but for a short 

 period under the total privation of food, while the excessive privation 

 of it produces effects upon the system which have not been often 

 observed with accuracy, but which are remarkably uniform, and highly 

 curious and instructive. Opportunities occasionally occur of noting 

 these effects with correctness and completeness, when, for example, 

 the passage to the stomach is closed up by disease ; or when, owing to 

 an unsound state of uu'nd, the individual refuses to take nourishment. 



During the first two or three days after the total abetii n< niu 

 food, in a person previously in sound health, the suffering from hunvi 

 is generally severe. The thirst is also at times distressing, but thirst 

 is not constantly attendant. The pulse during this period > 

 natural and so does the temperature of the body. All the evacuations 

 are scanty, and take place at distant intervals. After the first two or 

 three days the wasting of the body becomes visible, the fresh 

 characteristic of health disappears, and the features and the limbs, 

 instead of being plump and round, are sunk and collapsed. The loss 

 of weight, which increases rapidly, is appreciable, and the progress of 

 the emaciation is striking. The physical debility increases in exact 

 proportion with the emaciation : and the mind becomes weak, con- 

 fused, wandering, irritable, and at length almost deprived of reason. 

 All this time there is little or no pain from hunger or thirst, or these 

 uneasy sensations return only at intervals, and are seldom acute and 

 never lasting. The pulse at this stage may be a little quickened : it is 

 certainly easily excited ; and in like manner the heat, which seldom 

 sinks below the natural standard, is readily parted with, so that a 

 slight cliange of the temperature of a room is felt acutely, and produces 

 very uneasy sensations, a fact which demonstrates to the physician the 

 feebleness with which the functions are carried on, no less clearly than 

 the physical debility itself. The most remarkable and curious phi-no 

 nifim which next supervene, are those connected with the intellectual 

 faculties. The loss of power to perceive accurately, and to conn, rt 

 the trains of thought, is followed by decided delirium, which in at first 

 of a low muttering character, similar to that which takes place in tin- 

 last stage of typhus fever; but this sometimes passes rapidly into 

 furious and even maniacal delirium, requiring coercion, just as a 

 violent paroxysm of madness itself. Generally the delirium 

 ceded by a state of painful watchfulness and restlessness, it 

 impossible to procure sleep or quiet ; and, finally, the skin becomes 

 intensely hot, the pulse extremely rapid, the emaciation frightful, the 

 debility so great that scarcely the slightest movement can be performed, 

 and at length the individual sinks exhausted, commonly into a state 

 of stupor amounting to that complete and profound insensibility which 

 is technically called coma. 



This history- of the progressive changes which take place in tin- 

 system on the total abstraction of food, is illustrated in the most 

 perfect manner, by two cases which fell under the notice of physician!, 

 capable of accurately observing and duly appreciating each successive 

 event. Many wonderful stories are on record, of the truth of which 

 there U no sufficient evidence; but the cases to which HI- refer wen- 

 observed and recorded by men whose veracity is beyond question, and 

 who were endowed with more than ordinary discrimination and judg- 

 ment. The record on this account is invaluable, while in itself it is 

 highly curious and instructive. 



For the first case we are indebted to Dr. Currie, of Liverpool. In 

 August, 1796, a gentleman of Yorkshire, aged sixty-six, applied to this 

 physician for his assistance, on account of an obstruction in his 

 swallowing food, with which ho had been antic-ted for ten ur twelve 

 months. At first the complaint was slight ; it occurred only win -n ! 

 attempted to swallow dry and hard substances ; it afterwards ex< 

 to solids of every kind ; and, at the time he was first seen by his phy- 

 sician, although he was still able to pass down liquids, the quantity 

 he could swallow was not sufficient for his nutrition, and he wa con- 

 siderably reduced. On the introduction of a bougie into the gullet, 

 it passed about two inches cosily, but then met with an obstruction 

 which, by a moderate pressure, was overcome. It then passed easily 

 seven or eight inches more, but at the lower part of the tube, toward*-* 

 its tt-i initiation in the rardia. it met with a firm resistance, which no 



