HOW DISCOVERIES ARE MADE 119 



contained. He went on to discover how much heat is 

 required to convert a known weight of water into steam. 

 He found that about fifty-four times as much heat is 

 required as is necessary to heat the same weight of water 

 from the freezing-point to the boiling-point. But the 

 steam is no hotter than the boiling water ; hence Black 

 called this heat the ' latent heat ' of steam, because it lies 

 hidden in the steam and does not affect a thermometer. 

 Black made quantitative experiments that is, he not 

 merely made discoveries, but found the quantities in 

 which the changes took place. 



The way was now plain for Priestley and Scheele. They 

 heated all kinds of substances : if they evolved gas, that 

 gas was collected and examined ; but neither Priestley nor 

 Scheele paid much attention to quantities. The methods 

 of dealing with gases had to be invented, moreover. And 

 while Scheele caught his gases in bladders, Priestley in- 

 vented, or rather reinvented, what he called a ' pneumatic 

 trough,' a vessel filled with water containing jars and 

 bottles standing inverted full of water. If the tube lead- 

 ing from the retort in which the substance evolving the 

 gas was heated was directed so that its open end was 

 directly under the mouth of the bottle, the escaping gas 

 entered the bottle and displaced the water ; and when the 

 bottle was full, it could be corked, still under water, and 

 removed so that the gas could be examined. 



It is usually the case that discoveries have to be accom- 

 panied by inventions; the sequence is that to try any 

 new thing, a piece of apparatus has to be devised which 

 will effect the purpose or perhaps an apparatus already 

 known has to be altered so that it may almost be said 

 that invention and discovery go hand in hand. 



For this reason it is very important that the discoverer 

 should be a good worker in all kinds of materials in glass, 

 for most small pieces of apparatus can best be constructed 



