i 
ON ORGANIC COLOURING MATTERS. 491 
essential oils, ea. g., whose boiling points are much above that of water, are en- 
tirely dissipated in vapour if mingled with water raised only to the temperature 
of 212° F. Even bodies ranked, when anhydrous, among fixed substances, such 
as common salt, nitre, and boracic acid, rise with the vapour of water below its 
boiling point. It cannot be doubted that, in like manner, a temperature suffi- 
ciently low to hinder ice from volatilising in vacuo, or in still air, would not pre- 
vent it yielding a continuous stream of vapour in a current of gas. This power, 
indeed, of gases, as the more volatile bodies, to solicit and compel water-vapour 
to accompany them, is, at all temperatures, but especially at high ones, a formi- 
dable obstacle to rendering elastic fluids anhydrous. On the other hand, this dif- 
fusive power greatly increases the desiccating effect of gases, even not absolutely 
dry, when sent in currents over moist solids. 
How near an approximation may be made to perfect dryness in the case of 
gases, cannot be determined till we have a test of the anhydrous state applicable 
to elastic fluids. A criterion of some value would be the passage of a consider- 
able volume of the gas (ammonia excepted), through a weighed tube containing 
chloride of calcium, which should not increase in weight if the gas were anhy- 
drous; but, if the preceding observations are well founded, this test would cease 
to act before the gas was quite dry. 
[ have tried whether the change of tint which the so-called s mpathetic inks 
(solutions of the salts of cobalt and nickel) undergo when deprived of water, would 
Serve as an indication of dryness on the part of gases; but I find that it is a test 
of no delicacy. 
Indifference to colouring matter will certainly be found a negative indication 
of some value. Chlorine, ew. gr., which immediately bleaches, and sulphurous or 
carbonic acid, which reddens litmus, cannot be dry. This test, of necessity, is 
limited to the gases which affect organic colours, and would be useless in the 
case of oxygen, nitrogen, hydrogen, the carburetted hydrogens, &c. &c. 
A convenient way of examining the dryness of gases by means of colouring 
matters is to prepare, by blowing at short distances along a thin glass-tube, a 
series of small bulbs, in each of which a piece of litmus-paper may be placed. 
The papers are then to be dried in a current of air, passed through oil of vitriol 
and over chloride of calcium, and each bulb sealed off separately. In this way, a 
large number of bulbs can be prepared at the same time, and kept ready for use 
when required. One of these is to be placed in a tube forming part of the ar- 
rangement employed in the particular experiment, so that it shall be enveloped 
in the gas whose dryness is to be tested. By a sharp tap on the tube, the en- 
closed bulb is easily broken, and the gas and paper allowed to meet. There is no 
difficulty in making the bulbs thick enough to bear handling, and yet sufficiently 
thin to give way when required. 
In the preceding remarks, I have chiefly referred to the difficulty experienced 
VOL. XVI. PART IV. 6K 
