SCHEELE, PRIESTLEY, AND CAVENDISH 137 



bodies, and that these gases take just as essential and weigh- 

 able a part in chemical transformations as do other substances. 

 Black had already given us the first example of this, in 

 carbon dioxide. 



Thus the gaseous state of matter first became recognisable 

 as of full and general importance. Gases thenceforth 

 became continually the object of ever new investigation, 

 since these three workers had also developed the necessary 

 methods for dealing with them. The chief of these was 

 their enclosure in glass vessels over mercury, which neither 

 dissolves them as does water, nor contaminates them by its 

 vapour. 



The three scientists cannot be separated from one another. 

 They possess - as did Black also - the characteristic of not 

 quickly publishing in print what they discovered, and of 

 not even sending it, enclosed in sealed envelopes, to academies 

 (which proceeding obviously can only have a purely selfish 

 object); they allowed it to ripen in themselves as far as 

 possible, without however in the meantime hiding it com- 

 pletely from sight. It thus came about that each knew more 

 or less what the others were doing, and at times they worked 

 simultaneously on the same subjects, so that much appears 

 as their common achievement. 



In another respect also, the three investigators were 

 similar. The mass of new things they discovered contained 

 - for them obviously - a great quantity of unknown matter. 

 It is true that many gases were now known, some of them 

 combustible (hydrogen), some supporting combustion to 

 an astonishing degree (oxygen); and they could be weighed 

 with certainty, just like liquid and solid bodies. But the 

 phenomena of combustion, and the heat, and also the light, 

 that were produced, remained something completely 

 mysterious. The heat of course could even be measured 

 as regards its quantity by Black's method, like a substance 

 that could be weighed; but it was not known whether it 



