AT THE PHYSICO-TECHNICAL INSTITUTE 431 



' In thus undertaking to speak to you of these alterations in 

 the scientific standpoint, I must beg to make my excuses for 

 mentioning not only the most recent discoveries, but also many 

 that are old and familiar to you, which however, in the course of 

 time, have so entirely altered in significance that they appear 

 under quite a different aspect. 



1 1 may define the capital aim of my lecture by calling it a 

 discourse on the nature of substances, using this word, however, 

 in its older and wider sense. 



'According to modern use the word substance denotes ex- 

 clusively material bodies, which are located in definite points of 

 space, and can neither be destroyed nor added to ; which have a 

 definite mass, that is, a definite inertia in their motion, but are 

 also, as far as we know, subject to the universal force of gravita- 

 tion i. e. their weight is proportional to their mass ; and which 

 bodies are the carriers of unalterable forces, with which they 

 act upon other masses. In virtue of their mechanical forces and 

 inertia, i. e. their resistance to motion, they are perceptible to 

 our tactile sense ; they also affect our other senses upon many 

 occasions and in diverse ways, so that it has been, and is, easy to 

 learn their properties and the conditions of their rest and motion. 

 We know of a fairly large but still limited number of such in- 

 destructible substances, the chemical elements, and an ever- 

 increasing number of others which are compound, the chemical 

 compounds, which latter, however, are not incapable of being 

 destroyed or increased, since they are composed of the former 

 and can be resolved into them again. 



4 In its older sense the concept of substance was more 

 comprehensive. It corresponded more to the etymology of 

 the term id quod substat, that which subsists in the background, 

 or behind the mutable phenomena ; in Greek fj ova-ia, Being or 

 Essence, by which was understood not merely material things, 

 but the concepts of categories of things, subject to one common 

 law, of which, indeed, nothing very definite could be said, and 

 the attributes of which depended principally on the play of 

 fancy. The prototype which was for the most part adopted in 

 this was the human mind, and the principle that regulated 

 the processes in the living organism, the vital essence or 

 vital force. The nomenclature was governed by analogy 

 with the least material things that were known in the world. 



