90 BOARD OF AGRICULTURE. 



The attempt of Paracelsus (1540), and a succeeding school 

 of physicians, to explain most intricate physiological proc- 

 esses in the animal organism by the aid of the limited chem- 

 ical experience of their time, as well as by the assumption of 

 an ill-supported analogy of chemical and . physiological reac- 

 tions, caused only a temporary decline in the appreciation of 

 chemistry on the part of physiologists and phyi?icians. The 

 adoption of more exact modes of chemical observation 

 towards the close of the last century, not only restored the 

 former confidence in its teachings, but secured to chemistry, 

 in common with physics, a controlling influence on the de- 

 velopment of the more rational animal and vegetable physiol- 

 ogy of the present day. 



The introduction of the balance, by Lavoisier in 1783, 

 into the laboratory of the chemist, for the purpose of study- 

 ino: the chemical chang-es of all substances under their treat- 

 ment henceforth by weight, inaugurated the new era. A 

 careful use of this instrument soon demonstrated two impor- 

 tant facts : — 



First: Matter cannot be destroyed. 



Second: All chemical combinations are characterized by 

 definite proportions of their constituents. 



It became evident that, however thoroughly the various 

 qualities of a substance might be altered, one quality could 

 not be destroyed, neither by physical nor by chemical agen- 

 cies — namely, its weight; and also that under whatever 

 varying circumstances a certain definite chemical compound 

 might have been produced, the same relative quantity of its 

 elementary constituents is in every case required for its 

 production. To account for the entire original quantity of 

 the constituent elements of a substance throughout all its 

 various stages of transformation, became an indispensable 

 attribute of an acceptable explanation of an experimental ob- 

 servation in chemistry. The chemist of the new era, in the 

 language of the founder of modern chemical, animal physi- 

 ology, aimed at a more concise description of the observ^ation 

 and proposed to control the latter by number and weight. 



The speedy general recognition of the above stated facts 

 exerted soon a thus far unknown influence on many current 

 opinions regarding the true character of well-known changes 



