422 SCIENCE PROGRESS. 



unchanged even when all the other determining elements 

 vary within the possible limits imposed by the law. Thus 

 we perceive that the historical development of scientific 

 conceptions is ever associated with the discovery and 

 working out of such invariants ; in them we behold the 

 mile-stones which mark the track traversed by human 

 knowledge. 



Such an invariant of universal significance was found in 

 the idea of mass. This gives not only the constants of 

 the laws of Astronomy, but also appears no less invariable 

 in the case of the most deep-going changes to which we 

 can subject the objects of the outer world, namely, chemical 

 changes. Accordingly this idea appeared to be excellently 

 suited to form the central point of scientific law. Of 

 course it was too poor in connotation to serve alone for the 

 representation of the various phenomena, and had therefore 

 to be correspondingly extended. This was effected by 

 fusing with the simple mechanical idea the series of 

 properties which are associated with the property of mass 

 and are proportional to it. In this manner arose the con- 

 ception of matter, in which everything was summed up 

 that was associated in our sense-impressions with mass, 

 and which always accompanied it, as, for example, weight, 

 extension in space, chemical properties, etc., and the 

 physical law of the conservation of mass passed into the 

 metaphysical axiom of the conservation of matter. 



It is important to observe that, with this extension, a 

 great many hypothetical elements crept into a conception 

 originally free from hypothesis. For example, in the light 

 of this conception a chemical reaction had, contrary to 

 appearance, to be so considered that none of the matter 

 affected by the chemical change could possibly disappear 

 and be replaced by new matter with new properties. On 

 the contrary, the view required the assumption that when, 

 for instance, all the perceptible properties of iron and 

 oxygen had disappeared in iron oxide, nevertheless iron and 

 oxygen still existed in the body produced and had only 

 assumed other properties. At present we are so accustomed 

 to such an idea that it is difficult for us to conceive its 



