DISCOVERY BY MEANS OF INFERENCE. 591 



of silver, Sir H. Davy, in 1802, and also Mr. Wedgwood, 

 inferred the possibility of photography, and, by means of 

 experiments, succeeded in making some pictures on sur- 

 faces of chloride of silver. Stokes, also, in the year 1851, 

 inductively inferred the true explanation of the dark lines 

 in the solar spectrum ; and, in 1861, Bunsen and Kirchoff 

 confirmed its correctness by means of experiments made 

 with a spectroscope. 



The doctrine and discovery of latent heat was an in- 

 ference, made by Dr. Black in the year 1760, from the 

 results of his experiments ; and he made it known in his 

 subsequent lectures, but did not otherwise publish it. 

 Svanberg inferred that the temperature of space is 58 F. 

 below F. ; and M. Fourier arrived at the same inference 

 from different data. Prevost's theory of exchanges of heat 

 was also an inference from observed facts. It was by 

 studying the phenomena of heat that Seguin, in the year 

 1839, and Mayer, of Heilbronn, in 1842, inferred and 

 suggested the hypothesis of the mechanical equivalent of 

 heat, which Joule, by actual experiment, discovered ; 

 Mayer also attempted to discover it by calculation from 

 known data. 



Facts may, but great principles cannot, usually be dis- 

 covered without the employment of thought and reason. 

 It was by studying the early known facts of electricity, 

 and making new experiments in connection with them, 

 that Du Fay, about the year 1733, was led to in- 

 fer the existence of two kinds of electricity, and that 

 similarly electrified bodies mutually repel, and dissimi- 

 larly electrified ones mutually attract each other, and 

 published an account of his additional experiments, upon 

 which these principles are partly based, in the ' Memoirs 

 of the French Academy' in the years 1733, 1734, and 

 1737. He says: 'I discovered a very simple principle 



