Mr. C. J. Burnett on several Forms of Actinometer. 53 



the film of oil above it) be as convenient for photographic registry 

 as the column of liquid was ; and if in a metal one, photographic 

 (or other) registry might be carried out by the intervention of a 

 piston-rod and other easily contrived apparatus attached, or by 

 attaching an upright glass tube with a column of liquid, to the 

 top of the metal one ; and such a column of liquid, being not 

 fluctuating in height like those in our earlier-described forms, 

 but of one constant height, will give a correct register, as far as 

 absence of unequal pressure goes. (This suggestion of a metal 

 tube with a glass one over it, is made in case it should be found 

 difficult to fit the glass tube sufficiently well to the piston.) 



The second plan is the making our register (photographic, or 

 simply visible) dependent on the loss of weight experienced by 

 the gas-evolving liquid, the vessel containing the sensitive liquid 

 being either balanced by a delicate spring, or by a system of 

 leverage, on some plan allied to the steel-yard principle; or 

 else, and probably better, the vessel containing the sensitive 

 liquid is made to float at a fixed height (when charged) in 

 or above the surface of water or other liquid in a suitably- shaped 

 vessel, and the loss of weight, and consequently the amount of 

 actinic influence exerted, is measured by the height to which, 

 or rapidity with which, it rises after exposure or during exposure 

 to light. This form of apparatus has a considerable resemblance 

 to some of the hydrometers. The apparatus may of course be 

 arranged either so as to leave the sensitive-liquid-containing 

 vessel below, or so as to keep it above the liquid. Above should 

 have the advantage in the sensitiveness of the apparatus, by not 

 losing so much light by absorption; while under has the ad- 

 vantage of enabling us to have our thermometer in the water 

 close beside it, and so either to register the temperature, if we 

 wish, on the same sheet of paper on which the actinism is regis- 

 tered, or else to maintain the temperature of the sensitive liquid 

 uniform* by letting a constant flow of cold water pass through 

 the water vessel ; or rather, to prevent shakings and disturbance 

 of the registry, we may have the sensitive (-liquid) vessel, with 

 its connexions, protected from disturbance by the motion of the 

 current of cold water by an immediately surrounding glass vessel 

 or thin tube. The immersion of the sensitive vessel would also 

 be convenient, as enabling us to make many interesting obser- 

 vations connected with resistance offered to actinic rays, or mo- 

 difying effects produced on them, either by variously coloured, or 

 by colourless, liquids of various compositions, as solutions of 



* A set of observations with the sensitive vessel at different temperatures 

 would be in many ways likely to lead to interesting results, and could pro- 

 bably be easily managed by regulating the heat of the current passed round 

 the sensitive vessel. 



