CHEMISTRY. 



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P'.IY* 



I ig. 6. 



which pastes from the socket of the former to tliat of 

 the latter, and which i* made fast by meant of < 

 The screw thai tines this plate to the socket of the in- 

 ferior bar, ii ot such a length, as when screwed home, 

 to extend to a (mall distance beyond its inner surface, by 

 which mrans the apparatus, in descending along the up- 

 right bar, may be fastened to any part of it. 



To each of the tinned vetsrls (see Fig. 4.) two very 

 small mercurial thermometers are fixed, which arc of 

 different lengths; the larger reaching- as far as the centre 

 of the retkel; the shorter extending a little lower than 

 the surface. The thermometers a b and c d, which indi- 

 cate the hrat in the vessel GH, are applii J to a thin 

 plate of ivory </', which has two scales divided upon it, 

 one for each thermometer. This plate is fixed to the 

 vescel by means of a clamp A, the shank of which con- 

 sist* of two thin elastic brass plates, united at the top, 

 and reparable from each other at the lower extremity. 

 For the purpose of admitting the thermometers, the pro- 

 iecting part or, having the figure of a parallelo; 

 was added to the tinned vewtel. The exterior side of 

 this projection is received between the plates (if the 

 shatiks described above. The.,e plates are closely ap- 

 plied to it, by means of their elasticity, in consequence 

 of which the clamp is steadily retained in its position, 

 at the name time that it admits of a considerable <.'. 

 of elevation or depression. Hence the depth to which 

 the thermometers are sunk may be varied at pleasure ; 

 and, in order that this depth may be accurately known, 

 a scale of equal parts is divided upon the exterior plate 

 of the shank. Through the upper part of the shank is 

 passed a small male screw S, similar to that which is used 

 in adjusting the focus of a telescope. This is introduced 

 into a corresponding female screw, inserted in the ivory 

 plate, which has the thermometers fixed to it ; and as 

 that plate is suspended in the clamp by a pin, around 

 which it has a free motion, it is manifest, that, by turn- 

 ing the above mentioned screw, the thermometers may 

 be made to approach nearer the centre of the vessel, or 

 to recede farther from it. 



The two brass cylinders in this instrument were in- 

 tended to hold the gases experimented upon. The gas 

 to be used was first introduced into a tin syringe, pre- 

 viously filled with water. To the end of this syringe 

 was fixed a worm, terminating in a flaccid bladder. The 

 worm was surrounded by a mixture of salt and ice. The 

 gas was slowly forced through this worm from the sy- 

 ringe, and extended the flaccid bladder. The object of 

 this was to free the gas as much as possible from mois- 

 ture, by exposing it to the cold of the freezing mix- 

 ture. The brass cylinders being previously exhausted 

 by means of an air pump, the bladder was screwed to 

 the top of the stem, and the stop-cocks being opened, 

 the gas rushed in and filled the cylinder. 



The next object was to heat the cylinder containing 

 the gases equally, and without moistening them. Dr 

 Crawford's contrivance for that was the following: 

 AB (Fig. G.) represents a quadrangular tinned vessel, 

 that has the two smaller vessels, CD, EF, fixed to its 

 bottom by means of dovetails, along which they may 

 be made to slide, and may be so adjusted, that their dis- 

 tances from each other shall be equal to the distances of 

 the cylindrical vessels in which the air is contained ; 

 their size being such, that they shall be nearly accu- 

 rately filled by those vessels, when the latter are intro- 

 duced into them. It is therefore manifest, that warm 



water contained in the large vessel AB, and coming in- Chemieal 

 to contact with the external surfaces of the smaller ve- Apparatus, 

 sels CD, EF, will necessarily communicate a part of its *""""" 

 heat to those vessels, and to the cylinders which are 

 placed in them. If the cylinders had been heated by 

 being immediately immersed in warm water, a quantity 

 . i moisture would have adhered to their sides, which 

 would have produced an inaccuracy in the experiment. 

 Two , n, all tlu-rmomcters,/,r, an- introduced into the brass 

 cylinders (Fig. 4.), in order to indicate the heat which PLAT* 

 the gas contained in them requires. Each of the brass CXI. I. 

 cylinders is enveloped in flannel, in order to confine the t 'S- * 

 heat as much a* possible. 



Such is the ingenious apparatus of Dr Crawford. It 

 remains to say a tew words upon the way in which his 

 experiments were conducted. Suppose he wanted to 

 determine the relative specific heats i/f common air and 

 oxygen gas, he filled one of the cylinders with com- 

 mon air, and the other with oxygen gas. The cylin- 

 ders were both heated equally by the contrivance exhi- 

 bited in Fig. 6. They were then suddenly let down in- 

 to the tinned vessels, IK, GH, ( Fig. 4.) containing 

 each an equal quantity of water of the same tempera- 

 ture. They were allowed to remain for about 15 mi- 

 nutes, and being withdrawn, the water in the tinned 

 vessels was agitated for a little, and its temperature ta- 

 ken. As the weight and temperature of each brass cy- 

 linder and its flannel covering was the same, if there 

 w,<s any difference in the temperatures of the water in 

 the two tinned vessels, that difference must have beer, 

 owing to the heat communicated by the gas in the one 

 cylinder being greater than the heat communicated by 

 the gas in the other cylinder. Suppose, for example, 

 that the water exposed to the action of the cylinder con- 

 taining the oxygen gas, were one degree hotter than the 

 water exposed to the action of the cylinder containing 

 the common air ; it follows from this, that 37-91 cubic 

 inches of oxygen gas, when they cool a certain number 

 of degrees, give 1 more heat to a certain weight of wa- 

 ter, than the same bulk of common air. From the 

 knowledge of this fact, the capacity of each for heat is 

 easily inferred. Such was the ingenious method em- 

 ployed by Dr Crawford ; and the care and address with 

 which his experiments were conducted cannot be too 

 highly praised. At the same time, we are afraid that 

 his result" are not near approximations to the truth ; 

 and that the unavoidable errors of experiment, when the 

 whole difference between the temperatures amounts only 

 to one or two tenths of a degree, are such, as to render 

 every result obtained by his method of little value. 

 Though the time which a body takes in cooling, when 

 placed in the different gases, does not depend upon thc-ir 

 capacity for heat alone, yet as that capacity influence* 

 the result, valuable inferences may be drawn from expe- 

 riments made in that way ; and they promise to lead ul- 

 timately to a much more precise knowledge of the capa- 

 city of the different gases for heat, than any other me- 

 thod which has been hitherto thought of. 



The calorimeter is an instrument contrived by Lavoi- Calorinw- 

 sicr and La Place, for determining the specific heat of t- 

 bodies, and was first described by them in the Memoirs 

 of the French Academy for 1780. p. 355. Lavoisier 

 afterwards re-published the account of it in his Elements 

 of Chemistry, Part III. Chap. iii. It was founded 

 upon this well known fact, that if a hot body be sur- 

 rounded by a sphere of ice, none of the heat can pcnc- 



