Mr. C. J. Burnett on several Forms of Actinometer. 55 



nient j but if we estimate the weight in any other way, the dif- 

 ficulty disappears. 



With regard to all these modes of registry by gas-evolution 

 now described, there are two material deficiencies, at all events as 

 applied to continuous self-registry. The first is the constant 

 diminution of sensitiveness of the apparatus, consequent on the 

 diminishing quantity of the decomposable salt or salts left in the 

 liquid ; and this (though the inconvenience might possibly be to 

 some extent diminished by using a very large quantity of solu- 

 tion with only a small portion of it exposed to the light, and 

 arranged so as to secure a circulation of liquid, or by having a 

 supply of a concentrated solution of the salt or salts gradually 

 admitted, and there are also other possible contrivances), toge- 

 ther with the uncertainties about the influence of absorbed light 

 at the beginning of, and after withdrawal from, solarization, 

 about absorbed gas at the beginning of solarization, and about 

 so-called catalytic influence, or the impulse which changing par- 

 ticles give to others, renders it necessary to speak cautiously 

 about the value of at least continuous observations with one and 

 the same portion of liquid, till careful experiment has shown 

 what sort of results are attainable. 



As to repeated observations with successive and fresh portions 

 of liquid, we can of course feel much more confident of valuable 

 results, — photographic or other self-registry, if desired here, how- 

 ever, necessitating the recourse to rather more complex apparatus. 



In my old paper, the salts recommended for forming the sen- 

 sitive solution were those of ferric and uranic oxide, — the binoxa- 

 late, ammonio-oxalate, and soda-oxalate of ferric oxide, and the 

 binoxalate of uranic oxide being especially suitable. For conve- 

 nience, I tried also the mixture of some of the more generally 

 accessible salts, as uranic nitrate, or iron-alum, with oxalic acid or 

 binoxalates*. Some of my other experiments were with solu- 

 tions of chromic acid, or bichromate of ammonia, soda, or potash, 

 along with oxalic acid, or one of its acid or neutral salts. Citric, 

 tartaric, and other vegetable acids were tried instead of oxalic, 

 but they seemed in many cases to be much acted on even in the 

 dark. Solutions of brown oxide of manganese in tartaric, citric, 

 and, still far more, in oxalic salts were liable to the same objection, 

 as well as to that of uncertainty as to strength. A solution of 

 permanganate of potash along with one of these vegetable acids 

 or of its salts (or ammoniacal saltsf ?), however, seems not alto- 

 gether so unpromising {. 



* The presence of an excess of oxalic or other acid is perhaps useful, 

 t Nitrogen being liberated. 



t Solutions of ferric salts containing citric, tartaric, or other vegetable 

 acids, rendered alkaline by ammonia, and with the iron held in solution, 



