444 REPORT—1883. 
A glass flask of about 1-9 litres capacity was employed for the experiment, and 
within it was supported, by a piece of string, a glass tube 14 mm. in diameter, and 
filled with freshly distilled mercury, the flask being closed by a greased glass plate. 
After standing at the temperature of the laboratory for about nine days, the mer- 
cury tube was removed and a small quantity of boiling nitric acid poured into the 
flask and left to stand for some time. The acid was next neutralised by ammonia, 
and after the fumes in the flask had disappeared, the liquid was washed out with 
water, acidulated with hydrochloric acid, and treated with sulphuretted hydrogen. 
A slight brown colouration resulted. Several standard solutions of mercury were 
then made and tested with sulphuretted hydrogen in the same manner. ‘The 
liquid from the flask gave a deeper colour than the solution containing 00006 germs. 
of mercury, and a lighter colour than that containing ‘00012 erms. It may there- 
fore be assumed that the flask contained about ‘00009 grms. of mereury vapour. 
Subsequently the same flask was used and a tube of mercury 24 mm. in diameter 
(or exposing nearly three times as much mercury surface as the first), suspended in 
it and allowed to stand fora month. Treated in a similar manner the colour was 
nearly the same (a little lighter if anything) as that produced by a solution con- 
taining (00012 grms. of mercury. One litre of the air in the flask, therefore, con- 
tained -°S'}° = 00006316 grms. of mercury. As the theoretical weight of a litre of 
mercury vapour at 20° C, and the normal pressure is 8°3474 crms., the volume of 
the vapour in 1 litre of the air was :222216 1000 _ .997566 cubic centimetres or 
1 8 34 74 
qasveo Of the total volume. The pressure of the mercury vapour was therefore 
1g2, 460 I . 
32160 = 00574 mm., whereas Hagen’s number for 20° is ‘021 mm. 
It may be observed that this method might have been expected to give rather 
an excess than a defect of the quantity of mercury, in consequence of condensation 
of mercury on the sides of the flask, and although the experiment was of a some- 
what rough character, it seems to show that Hagen’s number is too high. 
A paper has also been published by Hertz (Ann. Phys. wu. Chem., N. F. xvii. 
193), in which he estimates the pressure of the vapour at 2U° to be only 0013 mm., 
or only about one-fifth as great as indicated by the foregoing experiments. 
10. On the Imperfection of the Galvanometer as a Test of the Evanescence 
of a Transient Current. By Prcfessor Lord Rayterau, I’. B.S. 
In certain electrical measurements a galvanometer is used to indicate whether 
or not the integral value of a current of short duration is zero. Tor example, in 
the method givenin Maxwell's ‘ Electricity,’ §755, for comparing the coefticients of 
mutual induction, M, of two pairs of coils, the evanescence of the integral current 
through the galvanometer is made the test of the fulfilment of a certain relation 
between the coefficients of induction and the resistances. The two primary coils 
are joined up in simple circuit with a battery. The two secondaries are also con- 
nected together in such a way that the inductive electro-motive forces conspire, 
and two points, P, Q, one on each connector, are brought into contact with the 
galvanometer terminals. In special cases, as for instance when the two pairs of 
coils are similar, there is no current through the galvanometer, whatever may 
happen in the primary circuit; but in general the establishment or interruption of 
the primary current will cause a deflection of the galvanometer indicative of the 
integral value of the current passing. The method consists in adding inductionless- 
resistance coils to one or other of the secondaries until this current vanishes. 
The required conditions are most readily obtained by supposing the galvano- 
meter circuit broken, and inquiring into the value of the electro-motive force E 
between the points P and Q. The same current y flows in both secondaries, and if 
« be the primary current, the equations are— 
ain 
dt dt 
+ dy de ‘ 
N,— +M,.— + Sy= —E 
“dt “dt 
N + Ry =1o 
