IN his notice of my book “Researches on the Affinities of the Elements” in NATURE, November 16, the reviewer impugns the legality of applying mathematical formulae to my surfaces. I trust I may be allowed to answer briefly my critic's objections. His difficulty as to the non-continuous nature is imaginary, and arises from a mistaking of the object to be achieved—which is simply to obtain either a surface or a mathematical expression from which can be deduced the affinities any one element exhibits for any other. This can be done from the formulae, and they do, therefore, characterise the chemical properties of an element which depend upon these affinities. Although there exist an infinite number of points on the surface which are occupied by no element, yet there exist only a finite number of points the x and y coordinates of which are whole numbers, and to every integer value given to x and y in my formulae there corresponds a definite element; so that, so long as we keep within the domain of integer numbers (as we are forced to do by the nature of the construction) continuity is attained.