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



gas-evolving or sensitive liquid, with, at the top, two openings, 

 one for a cork or stopper, and the other for the tube C which con- 

 veys the gas to the second reservoir B, but which does not de- 

 scend beyond the top of either reservoir. B is, as we have 

 already stated, the second reservoir ; and from the bottom of it 

 starts the register-tube D, which may either be continued directly 

 upwards, or slantingly, to diminish so far the pressure of the 

 liquid, or may be carried along horizontally, so as to diminish it 

 still further or eliminate it entirely, as in fig. 2, — the tube 

 with the scale of degrees thus resembling so far the common 

 register-thermometer (and if it should be necessary to prevent 

 the liquids flowing onwards, though I hardly think it will when 

 the tube is fine enough, we may have a short plug or cylinder 

 close to the surface of the liquid, just as in the thermometer 

 alluded to). 



Fig. 3 represents a plan (which will be found to answer well 

 enough without resorting to an additional reservoir and its accom- 

 paniment, just described) to prevent the mixture of the register- 

 liquid with the sensitive liquid ; and which is quite necessary 

 where oil is the fluid used for the former, as otherwise it would 

 come to the top of the sensitive liquid, instead of lying at the 

 bottom in contact with, and ready to be forced up, the register- 

 tube, as a heavier fluid might do. In fig. 3 the scale is most 

 conveniently attached to, and the registry made by, the side of 

 the tube next the reservoir; and the superfluous register-liquid, 

 as it is driven onwards, may then be allowed to flow out at the end 

 of the tube, which may be turned down so as to prevent its accu- 

 mulating in a column above, and acting by its pressure ; to get 

 rid of which source of error and of diminished sensitiveness, we 

 may mention (adhering still to the same general system of registry 

 by production of gas) two other plans. They are, first, the sub- 

 stitution (in an upright or other tube) of a solid, closely-fitting 

 cylindrical piston * for the varying column of the register-liquid 

 (the piston having, it may be, a little oil or other liquid above it 

 to show that there is no leakage of gas). Such a piston (or we 

 may call it a cylindrical plug, as it does not necessarily have any 

 rod attached to it), being constant and uniform in its height and 

 resistance, will, if well fitted and running smoothly, give a uni- 

 form resistance, and consequently be an accurate index of the 

 amount of gas liberated. If in a glass tube, it will (even with 



apparatus is in proportion to the size of the sensitive reservoir and the bulk 

 of its liquid exposed to sunshine. A good plan of facilitating the regular 

 rising of the liberated carbonic acid, is to put a little sand or broken glass 

 (of a given weight) into the sensitive liquid before solarization. 



* Say of some very light material, as pith, cork, gutta-percha, india- 

 rubber, to diminish resistance to the pressure of gas, especially when the 

 tube is upright. 



