ON WATKK AND ITS COMI'orNDS 55 



the reactions of many other substances, that it is impossible to describe 

 tin- majority of them at this early stage of chemical exposition. After- 

 wards \vc shall become acquainted with many of them, but at present 

 w.' shall only cite certain compounds formed by water. In order to 

 see clearly the nature of the various kinds of compounds formed by 



tin- weight df a cubic centimetre of steam at various temperatures. It has been shown by 

 experiment that the density of steam, which does not saturate a space, varies very 

 inconsiderably at all possible pressures, and is nine times the density of hydrogen under 

 similar conditions. Steam which saturates a space varies in density at different tem- 

 peratures, but this difference is very small, and its average density with reference to air is 

 OT>4. We will employ this figure in. our calculation, and will calculate what volume the 

 steam occupies at 100. One cubic centimetre of air at and 760 mm. weighs 



0'00r2'.)3 gram, at 100 and under the same pressure it will weigh or about 



1368 



tr()UO'.4(> gram, and consequently one cubic centimetre of steam whose density is 0'64 

 will weigh 0'000605 gram at 100, and therefore one gram of aqueous vapour will 

 occupy a volume of about 1,653 c.c. Consequently, the piston in the cylinder of 

 1 sq. c.m. sectional area, and in which the water occupied a height of 1 c.m., will be 

 raised l,f>r>3 c.m. on the conversion of this water into steam. This piston, as has been 

 mentioned, weighs 1,033 grams, therefore the external icork of the steam that is, that 

 work which the water does in its conversion into steam at 100 is equal to lifting a piston 

 weighing 1,033 grains to a height of 1,653 c.m., or 17'07 kilogram-metres of work i.e., is 

 capable of lifting 17 kilograms 1 metre, or 1 kilogram 17 metres. One gram of water 

 requires for its conversion into steam 534 gram units of heat or 0'534 kilogram units of 

 heat i.r., the quantity of heat absorbed in the evaporation of one gram of water is equal 

 to the quantity of heat which is capable of heating 1 kilogram of water 0'534. Each 

 unit of heat, as has been shown by accurate experiment, is capable of doing 424 kilogram- 

 metres of work. Therefore, in evaporating, one gram of water expends 424xO'534 = 

 (almost) '226 kilogram-metres of work. The external work was found to be only 

 17 kilogram-metres, therefore 209 kilogram-metres are expended in overcoming the 

 internal cohesion of the aqueous particles, and consequently about 92 p.c. of the heat or 

 work consumed goes in overcoming the internal cohesion. The following figures are 

 thus calculated approximately : 



Total work of External work of T , 



JVmpeniture evaporation in vapour in , "J " " 



Kiln-ram -metres Kiln -ram-metres woikol \apom 



255 13 242 



50 242 15 227 



100 226 17 209 



150 209 ly 190 



200 l'.)-2 20 172 



Thus it will be remarked from this table that the work necessary for overcoming the 

 internal cohesion of water in its passage into vapour decreases with the rise in tempera- 

 ture; this is in connection with the decrease of cohesion with a rise in tempera- 

 ture, and, in fact, the variations which take place in this case are very similar to those 

 which are observed in the heights to which water rises in capillary tubes at different 

 t lup.-ratures. It is evident, therefore, that the amount of external or, as it is termed, 

 useful work which water can supply by its evaporation is very small compared with the 

 am unit which it expends in its conversion into vapour. 



IP. considering certain physico-meclianical properties of water, I had in view not only 

 their importance for theory and practice, but also their purely chemical significance, for 

 it is evident from the above considerations that in even a physical change of state the 

 greatest part of the work accomplished goes in overcoming cohesion, and that chemical 

 cohesion, or affinity, is an enormous internal energy. 



