xiv.] UNITS AND STANDARDS OF MEASUREMENT. 321 



sun is iiot too large 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 Professor A. W. Williamson 

 has proposed as a convenient unit of volume in chemical 

 science, an absolute volume equal to about 1 1 -2. litres 

 representing the bulk of one gram of hydrogen gas at 

 standard temperature and pressure, or the equivalent weight 

 of any other gas, such as 16 grams of oxygen, 14 grams 

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

 any one of those gases which weighs as many grams as 

 there are units in the number expressing its atomic 

 weight. 1 Hofmann has proposed a new unit of weight for 

 chemists, called a crith, to be defined by the weight of one 

 litre of hydrogen gas at o C. and o76 mm., weighing 

 about 0-0896 gram. 2 Both of these units must be re- 

 garded as purely subordinate units, ultimately defined by 

 reference to the primary units, and not involving any new 

 assumption. 



Derived Units. 



The standard units of time, space, and mass having been 

 once fixed, many kinds of magnitude are naturally measured 

 by units derived from them. From the metre, the unit of 

 linear magnitude follows in the most obvious manner the 

 centiare or square metre, the unit of superficial magnitude, 

 and the litre that is the cube of the tenth part of a metre, 

 the unit of capacity or volume. Velocity of motion is ex- 

 pressed by the ratio of the space passed over, when the 

 motion is uniform, to the time occupied ; hence the unit 

 of velocity is that of a body which passes over a unit 

 of space in a unit of time. In physical science the 

 unit of velocity might be taken as one metre per second. 



1 Chemistry for Students, by A. W. Williamson. Clarendon Press 

 Series, 2nd ed. Preface p. vi. * Introduction to Chemistry, p. 131. 



T 



