234 



Mr. Frederick Guthrie on the Thermal [Jan. 21, 



I. " On the Thermal Resistance of Liquids." By Frederick 

 Guthrie, F.C.S. Communicated by Dr. Tyndall. Received 

 October 16, 1868. 



(Abstract.) 



The memoir of which the following is an abstract gives an account of 

 some experiments made by the author with the object of determining the 

 laws according to which heat travels by conduction through liquids. 



After pointing out the importance of the subject, and briefly recapitu- 

 lating the methods previously used and the results obtained by other ex- 

 perimenters, the " Diathermometer " is described. 



This instrument, which may be employed for the examination of the 

 thermal resistance or conducting power of solids as well as liquids, has the 

 following form. A hollow brass cone, having a platinum base, is screwed 

 with its apex downwards into a tripod-stand which rests upon adjusting 

 screws. The apex of the cone is tubular, and carries a cock, through 

 which passes a vertical glass tube graduated and dipping into water. 

 The level of the water in the tube is nearly as high as the apex of the 

 cone. By means of a micrometer screw, a second cone, exactly similar 

 and equal to the first, having its apex upwards, may be brought to any 

 required distance from the lower cone. The brass cones and their pla- 

 tinum faces are highly polished, and the latter are cleaned by washing 

 successively with hot nitric acid, caustic soda, alcohol, and water. The 

 upper surface of the lower cone is brought into an exactly horizontal posi- 

 tion, and the upper cone is lowered to any required distance from it. 

 There is thus formed between the platinum faces a cylindrical interval 

 of known height or thickness and diameter, and having its opposite faces 

 parallel and horizontal. This wall-less chamber receives the liquid whose 

 thermal resistance has to be measured. A liquid, introduced by means of 

 a strangulated pipette of known capacity (equal, say, to the interstitial 

 space when the cone-faces are 1 millim. apart) between the cones, remains 

 there by means of its adhesion and cohesion. A description is given of 

 the method used to get a constant current of water of uniform and known 

 temperature to pass through the upper cone. When such a current passes, 

 the platinum face of the upper cone becomes heated ; it communicates its 

 heat to the liquid in contact with it. The heat passes downwards through the 

 liquid, heats the upper surface of the lower cone, expands the air therein, 

 and depresses the level of the water in the tube attached to the lower cone. 



A description is given of the most prominent sources of error of this 

 instrument, and the means which were employed to eliminate them. It 

 is concluded, from direct experiments (1st, by measuring the time required 

 for the production of the first heat-effect in the lower cone ; 2ndly, by show- 

 ing the smallness of the difference caused by the introduction of ather- 

 manous disks), and from comparison with recent results of Magnus, that 

 the effect of radiation in all the cases tried is negligible if not nothing. 



By measuring resistance rather than conductivity, several sources of 



