242 PROGRESS OF SCIENCE. 



invented the four-prism SPECTROSCOPE, and both these 

 physicists were among the first to use this instrument in 

 CHEMICAL ANALYSIS. By means of it they discovered 

 caesium and rubidium (1860) a decisive step. Before Bun- 

 sen's researches on the chemical action of light in which 

 branch Roscoe did also valuable work there was no mode 

 of measuring the ENERGY OF LIGHT. Bunsen invented a 

 serviceable PHOTOMETER for measuring the different inten- 

 sities of light ; his work in photometry alone would ensure 

 him a high place among physicists. He invented also 

 an ice-calorimeter for determinations of heat in a small 

 quantity of substance. This was improved upon by Favre 

 and Silbermann, who invented another calorimeter which 

 furnishes a very delicate means of ascertaining the caloric 

 capacity of liquids, latent heats of evaporation, and the heat 

 disengaged in chemical actions. Bunsen made COUNTLESS 

 DISCOVERIES in the field of pure CHEMISTRY ; but his name 

 will always be remembered in association with KirchhofFs 

 in spectroscopy. 



1811 18 . Grove (Sir William) showed the CORRE- 

 LATION OF FORCES, one of the mighty laws of nature; 

 and latterly showed the permanence of the LAW OF 

 ANTAGONISM being thereby the discoverer of two great 

 generalisations. He also showed the production of heat 

 by magnetism, the ELECTROCHEMICAL POLARITY OF GASES ; 

 and decomposed water by heat alone. He invented the 

 electric battery which bears his name one of the most 

 powerful piles known ; also invented the gas battery for the 

 decomposition of some compounds such as water, iodide 

 of potassium, etc. 



1818 1889. Joule discovered the law of heat evolved by 

 voltaic electricity ; made a THERMOPILE which would indicate 

 a difference of heat of o*oooii4. But his chief work raised 

 him to the first rank among men of science, for he disclosed 

 and laid down one of the greatest laws of nature, viz., the 

 DYNAMICAL THEORY OF HEAT. The views of Locke, 

 Rumford, Davy, and even Carnot, as regards the production 

 of heat by motion, had no great influence on the progress of 

 scientific investigation. But in 1842, the French physicist 



