ON ORGANIC COLOURING MATTERS. 491 



essential oils, ex. gr., 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 sti-eam 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 aU 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 Aveight 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. 



1 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. 



IndiflFerence to colouring matter will certainly be found a negative indication 

 of some value. Chlorine, ex.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 coloiirs, 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. PAET IV. 6 K 



