Intelligence and Miscellaneous Articles. 407 



decreasing strength of the sensitive liquid, and also by the phenomena 

 which may be roughly characterized as persistence in chemical rest, 

 and persistence in a state of chemical change in the sensitive liquid. 

 Till the errors arising from these sources can be determined and 

 allowed for, such photographic registry can be of little use. By 

 operating, however, not continuously with one portion of liquid, but 

 with separate portions successively uncovered for a fixed period, and 

 again covered up by a proper clockwork or similar arrangement, all 

 difficulty as to registry is removed. Supposing, for instance, that 

 such a compound actinometer has been at work for a day of twelve 

 hours, registering successive hours or half hours, the observer has 

 only to note at the expiry of the twelve hours, or before refilling 

 and setting the instrument to work again, at what point the oil in the 

 register-tube of each of the twelve (or of the twenty-four) separate ac- 

 tinometers stands. Photographic assistance is hardly required.- Still, 

 for very compound instruments, intended to work for a long period 

 without attendance, it may be usefully employed ; and even in other 

 cases it is quite possible that the phenomena connected with per- 

 sistent rest and action of the sensitive liquid may render its employ- 

 ment advantageous ; and this can be readily managed, e. g., by making 

 the same clockwork which shuts up each separate actinometer, im- 

 mediately before it shuts it, uncover a slip of photographic paper 

 behind it. Another plan would be to have the full number, corre- 

 sponding to the periods to be registered, of sensitive-liquid reservoirs; 

 but instead of giving each its own register-tube, to have them all 

 communicating with one registry-liquid reservoir and register-tube. 

 This would simplify much the photographic registry, which would 

 be effected by making the register-tube, as elsewhere, close a slit, 

 and having the paper moved past behind it by a jerk at every half 

 hour or other period, corresponding with that at which the sensitive 

 reservoirs are closed. The exact amount of rise due to each reser- 

 voir's exposure would be thus shown on the paper. 



Second. Actinometry by means of a mixture of carbonic oxide and 

 chlorine having been in my paper only alluded to in passing, I may 

 now state that this mixture has some advantages over that of hy- 

 drogen and chlorine. In the first place we have no risk of explosion, 

 the gases uniting quietly even in sunshine. In the second place we 

 have a ready means of estimating the amount of action which has 

 taken place by the condensation effected, as the two gases, in com- 

 bining to form phosgene or chlorocarbonic acid gas, shrink to half 

 their previous volume. To render this condensation conveniently 

 visible, we have only to carry a tube from the reservoir containing 

 the mixed gases into a vessel containing eupion, or some similar 

 liquid, which will neither absorb nor in any way be acted on by 

 them, — the absorption being of course indicated by its rise, and the 

 exact amount of that being easily shown by the attachment of a 

 scale. It is, I think, unnecessary to describe the various modifica- 

 tions of this instrument which suggest themselves, or to say more 

 than that the same plans of combining a number of them into one 

 compound instrument for registry of successive intervals, which we 

 have indicated as applied to the sensitive -liquid actinometer, are 



