DISCOVERY BY MEANS OF NEW EXPERIMENTS. 529 



ordinary men might have looked through the prism under 

 similar circumstances, but, unlike Mams, would not have 

 detected anything new. 



Arago in 1811, by examining by polarised light, plates 

 of quartz cut perpendicular to the axis of the crystal, dis- 

 covered that the plane of polarisation of the light was 

 twisted either to the right hand or to the left, and thus 

 found what is now known as ' circular polarisation ' of light. 



It was by actual trial that Eobert Hooke discovered 

 that water could not be heated above 212 F. in an open 

 vessel. By experiment also, Boyle about the year 1645 

 discovered that warm water boiled rapidly in the rarefied 

 receiver of his air-pump. Papin in 1673 (and Huggins 

 in the year 1681), discovered that water would boil at a 

 much lower temperature in a vacuum than in an open 

 vessel ; and Dr. Cullen also rediscovered this fact many 

 years afterwards. Dr. Black, in the year 1760, by devising 

 and making the following experiments, discovered the 

 phenomenon of latent heat. He filled two glass flasks, 

 one with ice at C. (i.e. just warm enough to begin to 

 rnelt), and the other with an equal weight of water at 

 C. ; and suspended them in a room kept at 8'5 C. The 

 water acquired a temperature of 4 C. in half an hour, 

 but the melting ice remained at C., and occupied ten 

 and a half hours to become all melted and acquire a 

 temperature of 4 C,, or twenty-one times as long as the 

 other ; and as it had been taking in heat at least as fast 

 as the water, viz., four degrees each half-hour, it had 

 absorbed 84 such degrees, 80 of which must have been 

 absorbed during the melting. He made another ex- 

 periment, by means of which he found that one pound of 

 water at 79 C. would exactly melt a pound of ice at C. ; 

 the resulting mixture being entirely liquid water at C. 

 He also found that a pound of water at C., when mixed 



M M 



