146 



Mr. W. N. Hartley on the 



[Feb. 8, 



a tube of any size could be made to fit it, either by the india-rubber 

 stretching when the tube happened to be large, or by binding with a piece 

 of copper wire when it fitted loosely. The pipette was scratched with a 

 file at each end, and over the upper one was slipped a piece of india-rubber 

 tube, attached to a tube of glass about J inch in bore and 4 inches long, tightly 

 packed with cotton-wool. The caoutchouc tube was pinched by a burette 

 clip, and the extremity of the tube enclosed by the caoutchouc was broken 

 at the file-mark. The vacuum was considered good if the india-rubber tube 

 collapsed completely ; the burette clip was opened, and filtered air thus ad- 

 mitted into the vacuous space. In order to render any thing that might 

 be attached to the interior of the india-rubber tube harmless to the experi- 

 ment, it was dipped in glycerine and the glycerine squeezed out of it, or 

 treated in the same way with melted bees' -wax or paraffin. The pipette 

 fits into its place in the disk by means of the flexible stopper. By closing 

 the burette-clip, the tube can be broken at the lower point without more 

 than a drop or two of the liquid escaping. After about one third of the 

 liquid had been examined, one half of the remainder was allowed to run 

 into a flask which had been previously heated to between 200° and 300° C* 

 The tube was then removed, and the fine capillary point, when possible, 

 sealed at a gas-flame. The finer the point the more easily is this accom- 

 plished. A portion of liquid remains in the tube. On heating rather 

 strongly a little of this is driven out, and then no air can pass to the re- 

 maining liquid without passing over red-hot glass, which readily melts to- 

 gether. The tube and flask were then placed side by side in a warm place 

 to undergo further observation. If the tube, or class of tubes, were called 

 A, after opening it was labelled A', and the liquid out of it exposed to the 

 unfiltered air, A". The tubes and flasks labelled thus were kept in a cup- 

 board, the bottom of which was the metal lid of a long water-bath. It was 

 thought better not to place the flasks or tubes in water, because the aqueous 

 vapour which would thus surround the mouths of the flasks would create 

 an abnormal atmosphere which might or might not affect the experiments ; 

 besides, such a plan is not so cleanly. The objective made use of was 

 obtained from Messrs. R. and J. Beck. It was a J glass, without any 

 immersion-arrangement, and gave, with the second eyepiece of one of their 

 microscopes, a magnifying-power of 750 diameters. Occasionally, for con- 

 venience in drawing, a power of 420 diameters was employed. 



Method of examining a liquid which it was difficult to retain in the 

 pipette-tube. 



When it happened that the finely drawn-out end of the pipette was too 

 large to retain the liquid, it was allowed to run into a small glass vessel, 

 really a beaker cut down so as to measure about \\ inch in diameter and 

 1 inch high. Drops of the solution were removed from this to the glass 

 slides while it stood on the glass stage in the jar of hydrogen, by means of 

 * That is to say, baked in an oven the bottom of which was red-hot. 



