APPARATUS 85 



achievement would, however, never have been reached but for 

 the establishment of several important general principles which 

 were only discovered in the course of nearly a century of laborious 

 and sometimes dangerous experimental enquiry. 



In all the older text-books of chemistry a distinction was 

 made between the liquefiable or condensable gases and those 

 which were called "permanent gases." No doubt the more 

 philosophical writers on the subject foresaw from the close 

 resemblance between vapours and the more easily condensable 

 gases, such as sulphur dioxide and chlorine, that after all there 

 might be a similar relation between such gases and the so-called 

 permanent ones, and it was expected that had suitable power 

 been available the latter would prove to be also merely the 

 vapours of very volatile liquids. 



The difference between a vapour and a gas is now known to 

 be a definite physical difference, as will be explained presently. 



The history of the liquefaction of all the gases need not be 

 related in detail, but the following brief record will suffice to 

 show the position of the question in the former half of the 

 nineteenth century. 



The list of gases below is accompanied by the name of the 

 experimenter who succeeded in reducing them to the liquid 

 state either by compression alone, or by compression assisted by 

 the lowest temperatures then producible. 



Name of Gas. Observer. ' 



Sulphurous acid (Sulphur dioxide). Monge and Clouct, 



about 1800. 



Sulphur dioxide and Chlorine. Northmore, 1805. 



Chlorine, hydrochloric acid, sulphurs 



dioxide sulphuretted hydrogen, I Farad before Ig2g 



carbon dioxide, nitrous oxide, cya- j 



nogen, ammonia. J 



Ethylene, hydrogen iodide, hydrogem 



bromide, phosphoretted hydrogen, ^ , , . 



silicon fluoridefboron fluoride, arse- Farada y' before 1845 ' 



netted hydrogen. J 



Hydrogen, oxygen, nitrogen, nitric \ Remained unliquefi- 



oxide, carbonic oxide, marsh gas. J able. 



Experiments carried on by Thomas Andrews, Professor of 

 Chemistry in Queen's College, Belfast, after several years of 

 work gave the clue. In 1861 he described experiments in which 

 he had submitted some of these remaining gases to very great 



