PROFESSOR GRAHAM ON THE MOTION OF GASES. 
391 
resistance still further increased, and the highest of all, to bring out the fourth result 
or the law of temperatures. 
6. Finally, it will be remarked throughout, that transpiration is promoted by den- 
sity, and equally whether the increased density is due to compression, to cold, or to 
the addition of an element in combination, as the velocity of oxygen is increased, by 
combining it with carbon without change of volume, in carbonic acid gas. 
It was no part of my plan to investigate the passage of gases through tubes of 
great diameter, and to solve pneumatic problems of actual occurrence, such as those 
offered in the distribution of coal-gas by pipes. But I may state that the results must 
be similar, with truly elastic gases such as air and carburetted hydrogen, whether the 
tubes are capillary or many inches in diameter, provided the length of the tube is not 
less than 4000 times its diameter, as in the long glass capillaries of my early experi- 
ments. The small propulsive pressure applied to coal-gas is also favourable to trans- 
piration, as well as the great length of the mains ; and I should therefore expect the 
distribution of coal-gas in cities to exemplify approximately the laws of gaseous 
transpiration. The velocity of coal-gas should be 1‘575, that of air being 1, under 
the same pressure (p. 383). And with a constant propulsive pressure in the gaso- 
meter, the flow of gas should increase in volume with a rise of the barometer or with 
a fall in temperature, directly in proportion to the increase of its density from either 
of these causes. 
3 £ 2 
