1830.J The Progress of Physical Discovery. 19 



have constantly the same primitive molecule, and the same nucleus, on 

 which many others depend, remains still undecided, notwithstanding the 

 labours of Vauquelin, Biot, and Thenard ; and it is possible that the 

 primitive molecules of many substances, both saline and mineral, remain 

 still undiscovered. 



The true nature of chemical affinity, tables of which had been con- 

 structed by Geoffroy, in 1718, was recognized by the French chemist 

 Berthollet, in 1803. He first saw that there was no such thing as abso- 

 lute affinity, but that it was in fact a general tendency of any substance 

 to unite itself to others, whose force, with reference to each of the latter, 

 is measured by the quantity of them it can seize, and augment with its 

 own quantity ; that this forc would continue to act when three or more 

 substances are mixed, if it were not counterbalanced by opposite forces, 

 such as the indissolubility of one of the resulting combinations ; that it 

 is these latter causes which produce separations or decompositions ; and 

 that heat and pressure are two causes opposed to each other, which vary 

 affinity in different degrees. These views, the correctness of which has 

 been proved by a multitude of experiments, have a natural tendency to 

 connect chemistry with sciences, from which it was formerly isolated, inas- 

 much as the chemist, who is now obliged to consider accessory circum- 

 stances, and calculate their forces, cannot dispense with a knowledge of 

 geometry and general physics. And it is a thing by no means unim- 

 portant to the interests of science, that the minds of investigators should 

 be turned occasionally into other channels than that which they have 

 chosen for their peculiar studies, for it is only in proportion to the 

 variety of his knowledge that a man can possess adequate conceptions 

 of the unity and harmony of nature. 



Our knowledge of the phenomena of those chemical agents whose 

 materiality is not yet ascertained, such as light, heat, and electricity, 

 has, within the last forty years, been increased by scores of valuable 

 observations. We are especially indebted to Count Rumford for his 

 Inquiries, in 1799, into the Propagation of Heat by Friction, which seem 

 to favour the opinion that heat is a mere vibration of the molecules of 

 bodies. His thermoscope, and the differential thermometer of our coun- 

 tryman, Leslie, are of well known utility. M. Biot, by following up the 

 researches of Richman, Franklin, and Ingenhouz, on conductors of heat, 

 has established the law according to which it extends itself along the 

 length of bodies. The different manner in which heat is distributed in 

 liquids and solids, and the process by which, in liquids, the molecules 

 are displaced by dilatation, to make room for others which are heated in 

 their turn, are facts discovered also by Count Rumford, the conse- 

 quences of which are very great in the arts of domestic economy, build- 

 ing, and clothing. The sensation we call heat, indicates, in effect, that 

 we lose less caloric at a given moment than in that immediately pre- 

 ceding: thence the influence of bodies of various capacities, more or 

 less conductors of heat, and of different sorts of clothing. But a still 

 more important discovery was that of latent heat, first propounded by 

 Black, in his lectures delivered at Glasgow, who established the fact of 

 the retention, by every substance, according to its kind, of a certain por- 

 tion of heat which does not act upon the thermometer, and, consequently, 

 that bodies, whose heat might be marked by the same degree, would 

 differ often essentially in the caloric they contain. Black, and his dis- 

 ciple Irvine, Wilke, the Swede, and Delaplace, respectively laboured to 



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