336 SCIENTIFIC THOUGHT. 



of the change of velocity which they produce. One of 

 the great difficulties which stood in the way of the fixing 

 of these very simple mathematical relations and defin- 

 itions was the fact that all matter with which we can 

 experiment is under the influence of a constant but un- 

 known force, that which makes it fall if not supported. 

 It was only by freeing themselves from the effect of this 

 constant force, or by balancing it, that philosophers 

 gradually arrived at the conception and definition of 

 mass, or quantity of matter, as something independent 

 21. of its weight. It was reserved for Newton to show and 

 mass. ' define the exact relation which weight bears to the other 

 properties of matter defined and measured by his pre- 

 decessors. By doing so he added a new definition, a new 

 means of measuring the quantity of matter or its mass, 

 showing at the same time to what extent the popular 

 measure of matter i.e., its weight could be accurately 

 used for scientific purposes. Again, to express it in the 

 language of our day, Newton showed that matter is not 

 only that which offers resistance to change of motion, 

 but also that which causes change of motion in other 

 portions of matter : it is not only the object on which 

 force spends itself, it is the seat of this force, and the 

 degree in which it can change motion in other portions 

 of matter is proportional to the degree in which it 

 resists the change of its own motion in other words, 

 the gravity or weight of matter is proportional to its mass 

 or inertia, and is not dependent on any other difference, 

 whether of size or of quality. This second universal 

 property of matter, which brought out more clearly the 

 reciprocity of all mechanical, and subsequently of all 



