VALUE OF ACCURATE MEASUREMENTS. 393 



In order to obtain accurate ideas of substances and 

 forces, we require to know the exact degree of every 

 quality and property which the substance or force exhibits. 

 Nearly all the properties of substances and forces are of 

 degree only, and not complete : for example, no substance 

 is perfectly elastic, or an absolutely perfect conductor of 

 heat or electricity ; no one body is absolutely transparent 

 or opaque to light or heat ; all substances have different 

 specific gravities, and even the solid, liquid, and gaseous 

 states of matter merge into each other by insensible 

 degrees; and all these require to be measured under 

 varied conditions. Very few of the properties of forces 

 or substances are entirely characteristic nearly all of 

 them are relative, and possessed in different degrees by 

 several or many substances : for instance, the property of 

 attraction is not a characteristic of magnetism electricity 

 also exhibits it ; magnetism is not characteristic of iron, 

 because cobalt and nickel are also magnetic ; great specific 

 gravity is not possessed by platinum alone several other 

 of the noble metals are nearly equally heavy, &c. &c. In 

 the subject of chemical analysis we perhaps find the most 

 perfect examples of characteristic properties of bodies, but 

 even in this case a substance can rarely be distinguished 

 by means of a single property alone, but nearly always 

 only by several. 



The special degrees of the different properties possessed 

 by different bodies are much more frequently characteristic 

 of the different substances than are the properties them- 

 selves. With regard to most of the properties of matter, 

 each substance, in the pure state, usually possesses its own 

 special or characteristic degree of each property under the 

 same conditions of pressure, temperature, &c. : for instance, 

 the specific gravity of hydrogen, being assumed to be equal 

 to 1, that of nitrogen is 14, oxygen 16, chlorine 35 , &c. ; 



