IN NATURE of Dec. 11, 1926, p. 837, Prof. W. A. Bone stated that he would at some future date publish the results of experiments on the uniform movement of flame which disproved the law of speeds. The work referred to has now been published in the Proceedings of the Royal Society (A, 114, 420; 1927) and we can reply to Prof. Bone's letter. The principal mixtures he has chosen to test the law of speeds are of ethylene and acetylene with oxygen, and the choice is made because, to use Prof. Bone's own expression, these mixtures are so sensitive (i.e. highly responsive to accidental changes in experimental conditions).