The difference in temperature between the acetylene and the carbide in Caro's test accentuates the difficulty of gauging the heat in a carbide vessel by mere external touch, and supplies experimental proof of the previous assertions as to the low heat-conducting power of calcium carbide and of the gases of the decomposing vessel. It must not be supposed that temperatures such as Lewes has found ever occur in any commercial generator of reasonably good design and careful construction; they must be regarded rather as indications of what may happen in an acetylene apparatus when the phenomena accompanying the evolution of gas are not understood by the maker, and when all the precautions which can easily be taken to avoid excessive heating have been omitted, either by building a generator with carbide in excess too large in size, or by working it too rapidly, or more generally by adopting a system of construction unsuited to the ends in view. The fact, however, that Lewes has noted the production of a temperature of 807° C. is important; because this figure is appreciably above the point 780° C., at which acetylene decomposes into its elements in the absence of air. Nevertheless the production of a temperature somewhat exceeding 100° C. among the lumps of carbide actually undergoing decomposition can hardly be avoided in any practical generator. Based on a suggestion in the "Report of the Committee on Acetylene Generators" which was issued by the British Home Office in 1902, Fouché has proposed that 130° C., as measured with the aid of fusible metallic rods, [Footnote: An alloy made by melting together 55 parts by weight of commercial bismuth and 45 parts of lead fuses at 127° C., and should be useful in performing the tests.] should be considered the maximum permissible temperature in any part of a generator working at full speed for a prolonged period of time. Fouché adopts this figure on the ground that 130° C. sensibly corresponds with the temperature at which a yellow substance is formed in a generator by a process of polymerisation; and, referring to French conditions, states that few actual apparatus permit the development of so high a temperature. As a matter of fact, however, a fairly high temperature among the carbide is less important than in the gas, and perhaps it would be better to say that the temperature in any part of a generator occupied by acetylene should not exceed 100° C. Fraenkel has carried out some experiments upon the temperature of the acetylene immediately after evolution in a water-to-carbide apparatus containing the carbide in a subdivided receptacle, using an apparatus now frequently described as belonging to the "drawer" system of construction. When a quantity of about 7 lb. of carbide was distributed between 7 different cells of the receptacle, each cell of which had a capacity of 25 fluid oz., and the apparatus was caused to develop acetylene at the rate of 7 cubic feet per