222 
DR. TYNDALL ON MOLECULAR INFLUENCES. 
laws II' at the other. The cube, as before explained, does not touch the brass plate 
underneath it, but is supported on its four conical points, and the plate which bears 
these is itself as much as possible cut away to permit of a free circulation of air 
through the space efgh. Time is a precious commodity to the experimenter, and by 
the means described the cooling is hastened and the experiments can succeed each 
other more quickly. To hasten the cooling further I made use of a pair of small 
bellows during the first minute after the removal of each cube, and, afterwards, a 
plate of thin glass was placed over the junction, but not in contact with it. On the 
glass two drops of ether were suffered to fall from a pipette ; .its evaporation caused 
a refrigeration of the air underneath, which, in virtue of its increased density, sank 
and diffused itself around the place of junction. In this way the temperature at the 
junction was brought a little lower than that of the surrounding air; the needle of 
the galvanometer being thus brought back, not only to zero, but to a certain point 
at the other side of it; at this point the glass was removed and a new cube was 
introduced; the thermo-circuit was permitted to remain closed until the needle 
descended to zero, which it slowly did, when the cause of local cooling was remove : 
the thermo-circuit was then broken at and things stood as at the commencement 
of the former experiment. The voltaic circuit was once more closed, the current 
permitted to circulate sixty seconds, then interrupted by the left hand, the thermo- 
circuit being closed at the same moment with the right, and the first impulsion 
measured as before. 
When however these artificial means of cooling are adopted great care is necessaiw 
We must not use the bellows in some experiments and neglect the use of it in others . 
and if the ether be applied once, it must be applied throughout the entire series of 
experiments. It must continue to act for the same time, and the same quantity 
should be applied in all cases. Of course such precautions are only necessary when 
great accuracy is required, but here they are absolutely necessary. Judging fiom 
the description, the mode of experiment may appear complicated, but in reality it is 
not so. A single experimenter has the most complete command over the entire 
arrangement. The wires from the small galvanic battery (four of Bunsen’s cells) 
remain undisturbed from day to day ; all that is to be done is to connect the battery 
with them, and every thing is ready for experiment. 
There are in wood three lines at right angles to eaeh other, which the mere inspec- 
tion of the substance enables us to fix upon as the necessary resultants of molecular 
action : the first line is parallel to the fibre ; the second is perpendicular to the fibre, 
and to the ligneous layers which indicate the annual growth of the tree; while the 
third is perpendicular to the fibre and parallel, or rather tangent to the layers. 
From each of a number of trees a cube was cut, so that every two opposite faces were 
parallel to one of the above lines. Thus, two faces were parallel to the ligneous 
layers, two perpendicular to them, while the remaining two were perpendicular to the 
fibre. It was proposed to examine the velocity of calorific transmission through the 
