374 THE PRINCIPLES OF SCIENCE. 



Subsidiary Units. 



Having once established the standard units of time, 

 space, and density or mass, we might employ them for the 

 expression of all quantities of such nature. But it is often 

 found convenient in particular branches of science, to use 

 multiples or submultiples of the original units, for the ex- 

 pression of quantities, in a clear and simple manner. We 

 use the mile rather than the yard when treating of the 

 magnitude of the globe, and the mean distance of the earth < 

 and sun is not too ]arge a unit when we have to describe 

 the distances of the stars. On the other hand, when we are 

 occupied with microscopic objects, the inch, the line or the 

 millimetre, become the most convenient terms of expression. 



It is allowable for a scientific man to introduce a new 

 unit in any branch of knowledge, provided that it assists 

 precise expression, and is carefully brought into relation 

 with the primary units. Thus Prof. A. W. Williamson 

 has proposed as a convenient unit in chemical science, an 

 absolute volume equal to about 11*2 litres, representing 

 the bulk of one gramme of hydrogen gas at standard 

 temperature and pressure, or the equivalent weight of any 

 other gas, such as 16 grammes of oxygen, 14 grammes 

 of nitrogen, &c. ; in short, the bulk of that quantity of 

 any one of those gases which weighs as many grammes 

 as there are units in the number expressing its atomic 

 weight y . Professor Hofmann has also proposed a new con- 

 crete unit for chemists, called a crith, to be defined by the 

 weight of one cubic decimetre or litre of hydrogen gas 

 at o C. and o'76mm., weighing about 0*0896 grammes z . 

 Both these units if adopted must be regarded as purely 

 subordinate units, ultimately defined by reference to the 

 primary units, and not involving any new assumption. 



y ' Chemistry for Students/ by A. W. Williamson. Clarendon Press 

 Series, 2nd ed. Preface p. vi. * z < Introd. to Chemistry,' p. 131. 



