U7 



DIFFUSION. 



DIFFUSION. 



or throe minute needle-hole*, or aperture* of other shape. The most 

 remarkable instance observed bv Fraunhofer via that of a fine grating 

 through which wa* viewed a ilit of light parallel to the lint* of the 

 crating. In thi ewe spectra were formed to pure a* to exhibit the 

 luted UIM* ; and in thU way Fnunhofer has measured with the moit 

 extraordinary accuracy the ware length* corresponding to the fixed 

 lint* which he; denote* by C, D. E. F, 0, H. (Gilbert's 'Annolen.' 

 Bd. 74,*. 887.) Thi* class of phenomena ha* been moat elaborately 

 investigated by M. Schwerd (BeugungMnchcinungen.&c.V 



DIFFUSION. ThU term ha* a somewhat extended application 

 in modern science: thu* we speak of the iliffutiim of ligkt, aii'l i/ 

 knl ; the <li/ntiot> of gam and </ Hi/nidt : we also speak of a ilifurirt 

 partr. The diffusion of light i* intimately related to its rcfli-ction. 

 If we accept the optical definition of reflection, according to the law 

 of equal angle*, it would be impossible to account for the visibility of 

 object*: the geometrical reflection would perfectly represent to the 

 eye the source of tin- im-ident beam, but it would convey no idea of 

 the tire or colour of the reflecting surface. According to thin law 

 the image of the sun would be reflected from the surface of bodies 

 just M if they were perfect mirrors, but the bodies themselves would 

 not be seen : the moon would appear like n mirror reflecting the 

 image of the sun : the candles in a room would be reflected by the 

 wall* and other object*, and nothing would be seen but the multiplied 

 reflections of tbe Some. But as the surfaces of bodies are not perfectly 

 reflective, but on the contrary, contain innumerable roughnesses which 

 absorb a portion of the light and scatter the remainder, each rough- 

 ness become* the focus of a pencil of reflected light, the rays of winch 

 diverge equally in all directions, bearing with them the colour of the 

 reflecting body. Thus each point of the surface forms an independent 

 focus, by means of which each such point is made visible. This is called, 

 somewhat improperly, irm//ar rtjfrtctiva, but there is nothing irre- 

 gular in the phenomenon, for by means of it, light is diffused, and the 

 illuminated object* become OH it were secondary luminaries, and 

 render objects visible which are not within the direct influence of the 

 source of light, and these objects in their turn, by reflecting light 

 irregularly illuminate others, and in this way the light is reverberated 

 backward* and forward* so a* to render object* visible which are far 

 removed from any direct source of light. By means of this some 

 property the atmosphere diffuse* the light of the sun in all directions, 

 and produces the phenomena of twilight. Similar phenomena ore also 

 to be noted with respect to heat, although it is not cosy to determine 

 the effects with a* much precision. Radiant heat obeys the law of 

 equal angle*, but a portion of it that is not absorbed must be diffused 

 by irregular reflection. This subject will come again under our notice 

 in the article HEAT, and we content ourselves here with this plight 

 indication of it. 



The term difiuion as applied to gases and liquids, refers to that 

 process by which such bodies when in contact, pass through each other 

 and intermingle, although not necessarily related by chemical affinity. 

 ft noticed by Priestley, that if a closed bladder containing oxygen 



H 



ga* be placed in a jar of hydrogen gas, the jar and the bladder' will 

 after some time each contain explosive mixture* of oxygen and hydro- 

 gen. This phenomenon, however, is now referred to endotmotit, or 

 the otmotie f'/ree. [OSMOSE.] As an example of gaseous diffusion, we 

 may instance the case of chlorine and hydrogen, the one being thirty - 

 rix time* a* heavy a* the other, yet if a bottle of hydrogen be inverted, 

 and connected by means of a long tube with a bottle of chlorine, the 

 heavier ga* will in a few hours creep up the tube to mingle with the 

 hydrogen, and the light hydrogen will descend the tube to mingle with 

 the chlorine, and this mixture will be equal in every port of the 

 apparatus. Indeed, if sufficient time be allowed, this equal inter- 

 mixture will take place between all gases and vapours which do not 

 act chemically on each other. The rapidity with which this diffusion 

 occurs varies with the density of the gases, and the more widely they 

 differ in this respect, the more rapid is the diffusion. The rapidity 

 with which a light go* diffuses into a heavy one may be shown by 

 means of a diffutinn tubt, consisting of a gloss tube 12 inches long, one 

 end of which is closed with a dry porous substance, such as a plug of 

 plaster of Pari* : when thi* is filled with hydrogen gas at the pneu- 

 matic trough, and supported so that the water shall stand at the 

 same level i>n the inside and on the outside, water will be seen to 

 rise up the tube, and will continue to do so in opposition to gravity, 

 until in a few minutes, it will be some inches higher than the water 

 ouuide, in consequence of the gas escaping through the pores of the 

 phwter, and diffusing iUelf into the air more rapidly than the air 

 panes in and diffuses itaelf through the hydrogen. Experiments made 

 in this way show that the diffusirencts of a gas is in the inverse pro- 

 portion of the square root of it* density. The density of air being 1 , 

 the square root of that density = 1, and its diffusiveness = 1, the 

 density of hydrogen - (KMJB2, the square root of that density = 

 632, nd its difluuveneee (J^ 87994, or in an actual experiment 

 88. That i* to *ty, while one measure of air is passing into the 

 diffusion tube, 8-&S measure* of hydrogen ga* are passing out of it. 



When different ga*e* are introduced into the difliision tube, each 

 maintain* it* own rate of diffusion. The rate of diffusion is increased 

 by a rite of temperature, but not *o rapidly as the direct expansion by 



The process of diffusion perform* an important part in nature. 



" Accumulations of giu*.<. which are unfit fur the support of animal 

 and vegetable life, are by it* means silently and speedily dispersed, 

 and thi* process thereby contribute* largely to maintain that uni- 

 formity in the composition of the aerial ocean which is so essential to the 

 comfort and health of the animal creation. Respiration itself, but for 

 the process of diffusion, would fail of it* appointed end, in rapidly 

 renewing to the lungs a fresh supply of air in place of that which has 

 been rendered unfit for the support of life by the chemical change* 

 which it ha* undergone." (Miller. Kit-menu of Chemistry,' part i.) 



The escape of a ga* through a minute aperture into a vacuum is 

 called Errruox, which see ; and the passage of gaaea through capillary 

 tube* into a ratified atmosphere belongs to TRANSPIRATION, while the 

 passage of gases through diaphragm* must be referred to oufatflKMu. 

 [OSMOSE.] 



Liquids diffuse themselves among each other, except in those 

 few cases where they are not miscible, as in the case of oil and 

 water, or but little miscible, as ether and water. In mot OiiBn 

 however, the adhesion between the particles of dixsimilar fluids is 

 very |K>rfect, so that they become completely incorporated; BO much 

 BO, that an actual penetration of one body by the other seems to 

 take place, since the mixture occupies leas bulk than the liquids did 

 se|irately. Thus 100 part* of alcohol mixed with 100 of water 

 will measure only 196 parts, and the same; proportions of sulphuric 

 acid and water will measure only 1 85 part*. 



The phenomena of liquid resemble those of gaseous diffusion. \V. 

 have seen that hydrogen will descend to mix with chlorine, and 

 that this heavy gas will ascend to diffuse itself among tli< ; 

 of the hydrogen ; so also, if a tall glass jar contain at the bottom 

 one part of sulphuric acid, and over thin two parts of the blue 

 infusion of litmus, introduced so as not to mix with the acid, the 

 heavier acid will in two or three days have diffused itself through the 

 liquid, and the progress of the diffusion may be noted by the gradual 

 change in colour from blue to red. 



The subject of diffusion is greatly indebted to Professor Graham, 

 who has obtained some interesting result* by very simple means. Hin 

 apparatus consisted of a number of 4-oz. 'jars with openings in the 

 necks of 1*24 inch in diameter, for containing the trial solutions, and 

 a number of 30-oz. cylindrical vessels for containing the small jars. 

 The solution to be tried was poured into a 4-oz. jar to within half- 

 an-inch of the top ; it was then filled up with pure water, closed with 

 a glass plate, and let down into the cylindrical vessel, into which was 

 carefully poured about 20 oz. of distilled water ; the glass plate was 

 then cautiously removed, and the apparatus was kept undisturbed at a 

 steady temperature for several days. When the result had to be ex- 

 amined, the mouth of the solution jar was again closed and the jar 

 withdrawn from the vessel, the water of which was evaporated and the 

 salt that had entered into it thus determined by weight. 



From numerous experiments conducted in this way it was found : 

 I. That when solutions of the same substance, but of different degrees 

 of strength, were employed, the quantities diffused in equal times were 

 c<rttrit paribat proportional to the quantity in the solution. Thus in 

 4 solutions of common salt, containing respectively 1, 2, 3, and 4 parts 

 of salt per cent., it was found that in 8 days the quantities diffused 

 were, in "So. 1, 2'78 grains, in No. 2, 5'54 grains, or just double; in 

 No. 8, 8'87 grains, or treble, and in No. 4, 11 '11 grains, or four times 

 the amount diffused from the first solution. II. That the qua 

 of the substances diffused from solutions containing equal weight* 

 of different bodies, varied with the nature of the substance, as in the 

 following table. Each solution contained 20 ports of the solid in 

 100 parts of water, and was exposed for 8 days at a tertperature of 

 60-5 degrees. 



Sp. gr. of notation Weight in irraiiu 

 Substances used. at 60' diffused. 



Chloride of sodium . 

 Sulphate of m.lpnrHii 

 Nitrate of nods 

 Oil of vitriol 

 Rtifrar candy . 

 Barley suRnr 

 Starch sugar 

 Treacle of cane tugar 

 Gum arable . 



Albumen . . 



1165 



185 



120 



108 



070 



066 



OC1 



OG9 



060 



033 



S3-G8 



27-42 

 .11 '50 

 69-32 

 26-74 

 20-21 

 36.94 

 32-99 

 13-24 

 S-08 



Saline substances were found to arrange themselves in groups, the 

 members of each group being equidifl'ueive, and the rates of diffusion 

 in each group connected with the rate of diffusion of the other groups 

 by a simple numerical relation. Isomorphous salts had generally equal 

 rates of diffusion. Seven such groups were made out. In No. 1, were 

 hydrochloric, hydriodic, and hydrobromic acids, and perhaps also nitric 

 acid. These acids wcie found to be the mont ditlusible substances 

 known. Group 2, contains hydrate of ] otash and pro! aMy nmmonia, 

 Group 8, nitrate of potash, nitrat* of ammonia, chloride of potassium, 

 muriate of ammonia, and chlorate of potash. No. 4. nitrate of soda, 

 and chloride of sodium. No. 6, sulphate of potash, carbonate of 

 potash, sulphate f ammonia, and ferro-cyanide of potassium; pro- 

 bably also eliminate and bi-chromate, bi-carbonate, and acetate of 

 potash, and ferrid cyanide of potassium. No. 6, sulphate and carbon- 

 ,;!e of soda ; and No. 7, sulphate of magnesia. In the following table 



