CHEMISTRY OF THE NINETEENTH CENTURY. 623 



chemist. He was first engaged as assistant to Davy 'in 'tlie laboratory 

 of the Royal Institution in 1813, and afterwards In/the sarriQ-.place and 

 in the same capacity he was again engaged with Professor Brande. 

 Faraday's purely chemical discoveries bear no comparison with the 

 number and importance of those which give him an imperishable 

 fame in the history of electricity. One principle, however, of no, in- 

 significant general interest in chemical science, he did establish. 

 It had been always assumed, before his experiments were made, that 

 a vapour was something different in its nature from a gas. Faraday 

 showed that the difference is merely a question of temperature and 

 pressure, and in his skilful hand, many supposed permanent gases were 

 reduced to the liquid, and some even to the solid, form. It will afford 

 an illustration of the beautiful simplicity and rare skill which charac- 

 terize the appliances by which Faraday was accustomed to obtain his 

 results, if we explain his method of operation in some of his experi- 

 ments on the condensation of gases. His apparatus consisted of a 

 tube of very strong glass, 

 into which the materials for 

 generating the gas were in- 

 troduced, so as to occupy a 

 small space at one end, as 

 shown at A, Fig. 313. The 

 other end of the tube c was 



then sealed up, and the tube FJ G. 3 J 3- 



was then bent in the middle 



B by the application of heat. The portion of the tube B c was immersed 

 in a freezing mixture while heat was applied at the extremity A, in order 

 to liberate the gas. The gas, under the combined effect of cold and 

 the pressure exercised by itself, was in most cases condensed into a 

 liquid, occupying the end c of the tube. The means of measuring the 

 amount of pressure within the tube was equally simple. A globule of 

 mercury was introduced into a tapering capillary glass tube d e, and 

 the spaces it occupied successively in different parts of the tube were 

 marked thereon in black varnish ; in other words, the tube, narrow as it 

 was, was thus divided into spaces of equal capacity. The globule was 

 left in the wider end of the tube, and the narrower end e was sealed 

 up. The compression of the included air, as indicated by the position of 

 the quicksilver, thus became, in accordance with Boyle's law (p. 231), 

 the measure of the pressure on the included gas. Faraday succeeded 

 in liquefying, and in some cases solidifying, all the supposed permanent 

 gases except six, namely, hydrogen, oxygen, nitrogen, carbonic oxide, 

 nitric oxide, and marsh gas, which long resisted all the efforts made 

 to reduce them to the liquid state ; but in quite recent times M. Cail- 

 letet has succeeded in liquefying most if not all of these. The condi- 

 tion, liquid or gaseous, therefore, in which a substance exists, is known 

 to be determined by the circumstances of temperature and pressure. 



