306 report — 1879. 



neater air-bath heated by gas, but the chief merit in his method is that a tube 

 containing air is heated by the side of that containing the substance. As soon as 

 the substance is all converted into vapour, air is passed up into the second tube 

 until it occupies as nearly as possible the same volume as the vapour. After the 

 operation the air is measured at atmospheric pressure and temperature. 



Croullebois, in 1874, reverted to Bineau's method of using a large globe with a 

 long tube, but took the precaution to heat the upper portion in a water-bath. 

 Deville however, criticised his method rather severely, and pointed out that it was 

 an unwieldy apparatus to manipulate. 



In 1868, Hofmann, in modifying the Gay-Lussac method, while he adopted the 

 lono- tube which had been previously used by Bineau, Playfair and Wanklyn, and 

 Grabowski, introduced such an important alteration into the apparatus that it is 

 not spoken of as his modification but as his method. Instead of heating the 

 substance-tube by a water-, oil-, or air-bath, he simply enclosed it in a, slightly 

 larger mantle tube, and passed the vapour of a liquid of definite boiling-point 

 through the intervening space, selecting the liquid according to the temperature 

 required. By this means he not only rendered the apparatus much more compact, 

 but he maintained a steady temperature with the greatest ease. Wichelhaus, 

 in 1870, anxious to avoid the uncertainty introduced by the doubt as to the 

 temperature of the column of mercury between the bottom of the outer tube and 

 the trough, dispensed with the latter by fixing to the lower extremity of the 

 substance-tube an inverted siphon containing mercury. Then, by lengthening and 

 suitably enlarging the lower extremity of the outer tube, the whole of the inner one 

 can be surrounded by vapour. 



Grabowski, in 1875, in order to obtain a high temperature, employed the vapour 

 of naphthalene as the heating medium in using Hofmann's apparatus ; but Engler, 

 in the following year, finding that the stoppage of the tubes from the solidification 

 of the condensed hydrocarbon was troublesome, proposed to obviate the difficulty 

 in the following manner : — He fixed to the lower end of the outer tube a metal 

 socket provided with a short side-tube similar to those used for heating funnels. 

 Then, by boiling the heating medium in this tube and allowing the vapour to 

 cohobate in the space between the two glass tubes, he dispensed with all the 

 arrangement of flask, tubes, and condenser. 



Hofmann at the same time made several modifications in his apparatus : — 

 (1.) He proposed heating the whole length of the inner tube by making the 

 outer one long enough to enter the mercury in the trough, and provided for the 

 escape of the condensed liquid and excess of steam by having a small side-tube 

 affixed a short distance above the level of the mercury. 



(2.) Finding that graduated tubes were very liable to crack, he proposed using 

 plain ones in the following manner : — In the bottom of the mercurial trough he 

 placed a piece of sheet india-rubber attached to an iron plate and provided with a 

 groove on its upper surface ; the iron plate was furnished with a handle. During 

 the heating the inner tube stood over the groove to allow of the escape of the 

 mercury. When the level became stationary, communication with the mercury in 

 the trough was cut oif by shifting the india-rubber disc until the inner tube rested on 

 the flat surface. The height of the column in the inner tube was then noted by 

 means of a cathetometer ; the outer tube was then removed, and a gummed label 

 attached to the inner one to indicate the mercury level. After cooling, the volume 

 of the vapour is determined from direct measurement. 



(3.) In order to avoid the cracking of the tubes in cases where liquids of high 

 boiling-point were used, he proposed connecting the lower end of the outer tube 

 with the inner one by a cork, through which two tubes leading to the flask or 

 boiler passed. One of them led below the liquid, while the other, which was pro- 

 vided with a stop-cock, reached only just below the cork. If this stop-cock be 

 closed while the liquid is being heated a portion of it is forced up the space between 

 the two glass tubes, and thus the mercurial column is heated more gradually. 

 When the liquid reaches the boiling point the stop-cock is opened and the circula- 

 tion of the steam proceeds as usual. The upper part of the outer tube must be 

 sufficiently elongated or provided with a small tube leading to a condenser. 



