plate, from which all modern photography has 

 developed. The daguerreotype was announced 

 in 1839. 



D'Alembert, Jean le Rond, iii, 15. Born at 

 Paris, 1717; died at Paris, 1783. French mathe- 

 matician and philosopher. Educated at the Col- 

 lege Mazarin, and after trying law and medicine, 

 devoted himself to mathematical science. An- 

 nounced his famous principle of the equality of 

 impressed and effective forces, 1743. Made many 

 astronomical and mathematical solutions. 1751, 

 with Diderot undertook editorship of the Ency- 

 clopedic. Withdrew in 1758. Had offers from 

 foreign rulers which he did not accept. 



Daniell, John Frederick, iii, 236. Born at Lon- 

 don, 1790; died at London, 1845. English physi- 

 cist who gave special attention to chemistry and 

 meteorology. Professor of Chemistry, Kings 

 College, London, 1831. Established, with Pro- 

 fessor Brande, the "Quarterly Journal of Science 

 and Art," 1816. Invented the hygrometer about 

 1820. 



Dalton, John, iv, 40. Born at Eaglesfield, 

 England, 1766; died at Manchester, 1844. Eng- 

 lish chemist. 1793 was made Professor of 

 Mathematics and Physical Science at Manches- 

 ter. He developed the atomic theory in chemis- 

 try, which revolutionized the science. In phys- 

 ics he experimented with the force of steam 

 and electricity and expansive force of gases. 



tWas held in the highest distinction and was the 

 recipient of many honors. Was color-blind, and 

 gave scientific description of this malady, which 

 is often called Daltonism. 

 Dana, James Dwight, iii, 162. Born at Utica, 



