Mr. C. J. Burnett on several Forms of Actinometer, 51 



of plagiarism against Dr. Woods, it appears right that you and 

 your readers should know that the measurement of actinism by 

 the gas evolved is not altogether so novel an idea as Dr. Woods 

 imagines. By reference to my old sketch alluded to, you will 

 find that not only the principle of measurement by gas-evolution, 

 but more also of what Dr. Woods brings before the public as 

 novel, has been anticipated ; and had my sketch not been a very 

 hurried one, abridged hurriedly from my old notes of 1855, it 

 would have contained various other particulars which are, I 

 believe, even yet new to the public, — and among others the fol- 

 lowing modifications, &c, which may perhaps be of interest. 

 I remain, Gentlemen, 



Your obedient Servant, 



C. J. Burnett. 



A column of oil, spirit, or other lighter liquid may be made 

 {entirely or partially) to replace the mercury, the employment of 

 this lighter liquid enabling us to use an erect register-tube with- 

 out having the result so seriously interfered with by the pressure in 

 it, as is the case where such a very heavy register-fluid as mercury 

 is employed. The interference of the pressure of a column, and 

 especially one varying in weight according to its varying height, 

 gives rise, be it noted, to two inconveniences : first, the render- 

 ing of equal distances on the register-scale not true measures of 

 the quantity of gas evolved, and of its pressure ; and secondly, to a 

 smaller and constantly decreasing sensitiveness of the instrument, 

 owing to the smaller distance which the column, of the heavier 

 liquids especially, is pushed forward as its height and pressure 

 increases. There is an objection to water as a registry-liquid, 

 that it absorbs and transmits a large portion of the carbonic acid, 

 which neither oil, mercury, nor even, 1 think, spirit does. The 

 employment of the sensitive fluid itself for an index, as suggested 

 and employed by Dr. Woods, is manifestly still more objection- 

 able, from the chemical action in this case going on in the register- 

 tube as well as in the reservoir. 



The oil or other liquid should be coloured, say dark red, to 

 make the results more legible, or to facilitate the photographic 

 registry of them, where that is wished to be carried on. 



Using either the heavy or the lighter liquids, the apparatus, 

 as figured formerly in one of its most simple states, may be with 

 advantage modified in very many ways : e. g. Plate I. fig. 1 repre- 

 sents an apparatus in which the registry-liquid is contained in a 

 second bottle or reservoir, A being the reservoir* containing the 



* No attempt at correct proportion is made in these rough figures, 

 which are only intended to show the principle of action. Thin glass, to 

 intercept little light, would be advisable. N.B. The sensitiveness of the 



E 2 



