144 THE SCIENTIFIC PAPERS OF 



platinum wire of '009 inches diameter, which had been prepared, 

 by the old welding process (which gives, as already stated, a much 

 more conductive, and therefore, a purer wire than the more recent 

 process by fusion in a Deville furnace). In one of the series of 

 experiments, this wire was wound upon a cylinder of pipeclay, in 

 helical grooves to prevent contact between the convolutions of the 

 wire. To arrive at a knowledge of its electrical resistance, when 

 subjected to various temperatures, I placed it, together with a 

 delicate mercury thermometer made for me by Messrs. Negretti 

 and Zambra, in a copper vessel, contained in a bath of linseed oil, 

 which (in order to prevent the too sudden radiation of its heat, and 

 consequent variation of temperature) was placed within a larger 

 vessel, the space between the two being packed with sand. The 

 leading wires of the platinum-coil were then connected with a 

 Wheatstone's balance and a delicate galvanometer. The bath was 

 very gradually heated by a series of small Bunsen's burners, and 

 whilst the oil was kept in continual motion, the resistance of the 

 platinum wire was read off at intervals of 4 or 5 Centigrade. 

 When the highest point had been reached, the bath was allowed to 

 cool down gradually, and measurements were taken at the same 

 points of temperature as before. This was repeated several times, 

 until about six readings of the resistance of the wire at each point 

 of temperature had been obtained. The mean readings are 

 contained in the first table given at the end of this Part. The 

 platinum wire was carefully annealed, and maintained for several 

 hours at the maximum heat before the observations were 

 taken. 



Not satisfied with this single series of experiments, I undertook 

 a second series under somewhat different conditions. Instead of 

 coiling the wire upon a pipe-clay cylinder, I employed a spiral 

 contained in a glass tube and hung by its leading wires in a 

 rectangular air-chamber, about 6 inches long, 3 inches broad, and 

 3 inches deep, the space between the walls being filled with sand 

 to insure a very steady temperature inside. Three mercury 

 thermometers were inserted through the cover of this double 

 chamber, so that their bulbs stood around the platinum coil in the 

 same horizontal plane. This box was heated externally, by five 

 small Bunsen's burners, a gas pressure regulator being applied to 

 give steadiness of heat. Irregular losses of heat by radiation, or 



