50 PRINCIPLES OK CHEMISTRY 



of potassium permanganate does no harm, because in the next distilla- 

 tion it is left behind in the distillation apparatus. The next distilla- 

 tion should then be from a platinum retort with a platinum receiver. 

 Platinum is a metal which is not in any way changed either by air or 

 water, and therefore nothing passes from it into the water. The water 

 obtained in the receiver still contains air. It must then be boiled for 

 a long time, and afterwards cooled in a vacuum under the receiver 

 of an air pump. Pure water on evaporation does not give any sedi- 

 ment, does not in the least change, however long it be kept, and if air 

 have no access to it does not putrefy like water only once distilled or 

 impure ; and it does not give bubbles of gas on heating, nor does it 

 change the colour of a solution of potassium permanganate. These 

 are a few signs by which the complete purity of water may be recog- 

 nised. 



Water, purified as above described, has constant physical and 

 chemical properties. For instance, it is of such water only that one 

 cubic centimetre weighs one gram at 4 C. -i.e., it is only such pure 

 water whose specific gravity equals 1 at 4 C. 9 Water in a solid state 

 forms crystals of the hexagonal system 10 which are seen in snow, which 



9 Taking the generally-accepted specific gravity of water at its greatest density i.e. 

 at 4 as 1 it has been shown by experiment that the specific gravity of water at different 

 temperatures is as follows : 



At - 5 D . . . 0-99929 At 30 ... G'99577 



., ... 0-9JHIS7 40 . . . 0-99230 



,,4-10 . . . 0-99974 50 . . . 0'98817 



15 ... 0-9991C) 80 ... 0-97192 



20 0*99820 100 : 0-9.VO4 



Water at 4 is taken as the basis for reducing measures of length to measures of 

 weight and volume. The metric, decimal, si/stem of measures of weights and volumes is 

 generally employed in science. The starting point of this system is the metre (39'37 

 inches) divided into decimetres ( = 0'1 metre), centimetres ( = O'Ol metre), millimetres 

 ( = O'OOl metre), and micrometres (- one millionth of a metre). A cubic decimeti'e is 

 called a litre, and is used for the measurement of volumes. The weight of a litre of 

 water at 4 in a vacuum, is called a kilogram. One thousandth part of a kilogram, or one 

 cubic centimetre, of water weighs one yratn. It is divided into decigrams, centigrams, 

 and milligrams ( = O'OOl gram). An English pound equals 453'59 grams. The great 

 advantage of this system is that it is a decimal one, and that it is universally adopted in 

 science and in most international relations. All the mecuures died in thin ii-orl; (in- 

 metrical. The units most often used in science are : Of length, the centimetre ; of 

 weight, the gram ; of time, the second ; of temperature, the degree Celsius or Centigrade. 



10 As solid substances appear in independent, regular, crystalline forms which are 

 dependent, judging from their cleavage or lamination (in virtue of which mica breaks 

 up into laminae and Iceland spar, &c., into pieces bounded by faces inclined to each other 

 at angles which are definite for each substance), on an inequality of attraction (cohesion 

 hardness) in different' .'directions which intersect at definite angles ; therefore, the 

 determination of crystalline forms offers one of the most important external marks 



