IOWA ACADEMY OF SCIENCES. 23 



theory of evolution back to the time of the early Greek philos- 

 ophers, yet neither assumed a scientific form until about the 

 beginning of this century. In these and all similar cases the 

 speculations of philosophers, suggestive though they have 

 been, will be passed over and the origin of theories placed at 

 the times when they arose as the results of true scientific 

 research. 



It is assumed that there is no occasion in this presence to 

 explain any prominent fact or theory of science farther than to 

 mention it by name. 



CHEMISTRY. 



The phenomena of Chemistry that appeal to ordinary obser- 

 vation are not many, and the factors of chemical reactions lie 

 beyond the reach of the senses. This may account for the fact 

 that scientific chemistry is a matter of recent origin. Its 

 beginning as an empirical art prjbably antedates authentic 

 history, but as a science it is difiicult for one of the present 

 time to conceive of its existence prior to the discovery of 

 oxygen by Priestley in 1774, and the explanation of the relation 

 of this most important element to combustion and calcination 

 by Lavoisier from about 1777-1783. 



The history of Chemistry down to Lavoisier is, as regards 

 theory, a long night with only here and there small gleams of 

 light due to the illumination of the torch of a Boyle in the 

 seventeenth, and a Black in the eighteenth century. The 

 former clearly distinguished elements and compounds, and 

 gave the beginnings of a theory of chemical reactio o ; but his 

 good work was lost sight of, and completely disappeared with 

 the rise of the theory of phlogiston by Stahl, in the seventeenth 

 century. This theory has such an important relation to the 

 material theories of light and heat that a word of explanation 

 is necessary. It assumed a fire principle which escaped in the 

 combustion and calcination of bodies. A calx, or oxide in our 

 language, was, therefore, an element, while a metal was due 

 to the union of a calx with phlogiston. The more violently a 

 body burned the richer it was in phlogiston, the gaseous pro- 

 ducts of combustion seeming to be ignored. Later in the history 

 of the theory, carbon, sulphur and hydrogen were successively 

 identified with phlogiston. At first no account was taken of 

 the increase of weight when a metal changed to a calx. Later 

 when hard pressed upon this point phlogistoaists did not hes- 

 itate to ascribe to phlogiston the property of negative gravity 



