OBSERVATIONS OF CHANGE OF TEMPERATURE 37 



subdivided into 100 equal lengths. A thermometer with a 

 scale of this kind is said to be centigrade. 



When a portion of matter is at such a temperature that 

 the mercury in a thermometer, placed so as to be in thermal 

 equilibrium with it, stands opposite a certain number on the 

 scale, that portion of matter is said to have a temperature 

 of that number of degrees. For example, if the level of the 

 mercury is at 15 the body is said to have a temperature of 

 15 degrees centigrade (or 15 C.). There is no difficulty in 

 perceiving that we add nothing, so far, to our knowledge, 

 when we say that a body is at 15 C. temperature. The 

 thermometer is an instrument which enables us to say when 

 two bodies are in the same thermal condition, on the assump- 

 tion that similar causes produce similar effects. This assump- 

 tion will afterwards be found to be reasonable. We cannot, 

 however, yet consider that these numbers or degrees afford 

 anything more than a rough comparison of different thermal 

 conditions, nor is it likely that they give any correct informa- 

 tion about the real basis of temperature. 



By the use of the balance we are able to decide when two 

 quantities of matter are equal, and by the use of the ther- 

 mometer we can judge when two bodies are at the same 

 temperature. But by the use of the balance we can com- 

 pare two quantities of matter, by collecting together a sufficient 

 number of sufficiently small standard quantities to produce in 

 each case the same effect. The numbers then indicate the rela- 

 tion in quantity. In the case of temperature, on the other 

 hand, we have at present no means of estimating quantity, 

 and we are face to face with a different order of phenomena. 

 Equality of temperature, just like sameness of colour, has 

 no apparent connection with quantity. We begin now to deal 

 with a condition or quality which cannot be isolated from 

 matter, and which cannot be divided into parts or added 

 together. We can, however, observe that changes of tempera- 

 ture are determined both by the quantity and the kind of 

 matter in which they take place. By combining the concep- 

 tion of temperature with that of matter we may regard a 

 change of temperature as a measurable quantity. 



