1919] PERIODIC PRECIPITATES 661 Hookham's Crystals. Another remarkable example of fine periodic structure was brought to my notice by Mr George Hookham. In this case double refraction plays an important part and a careful study of the crystals requires the use of a polarizing microscope. I have had the advantage not only of receiving interesting specimens and a sample of one of the solutions employed, but also of witnessing for myself Mr Hookham's procedure. The active ingredient is copper sulphate; but, as it is desired to obtain a film which is initially amorphous, other ingredients must be added. In the solution given me there is both salycine and sugar. Mr Hookham describes it as consisting of a solution saturated (in the cold) with copper sulphate and salycine, to which is added 3 per cent, of strong syrup. A few drops are placed upon a strip of glass, such as are ordinarily used for microscopic slides, and are spread with the finger. The slide is then warmed over a spirit lamp, when any excess of liquid may be thrown off. By a further application of heat the whole is then dried somewhat rapidly. There is usually immediate formation of crystals at the edges, but throughout a space in the interior the film should be amorphous and nearly invisible. At this stage the amorphous film shows nothing in the polariscope, but in a short time after cooling developments set in and proceed with rapidity. There is much here to excite admiration and perplexity, as in other similar phenomena of crystallization, but the feature in which I am specially interested, viz. the formation of a structure periodic several thousand times in the inch, does not appear to present itself unless the plate is kept warm until crystallization has set in. Mr Hookham mentions a temperature about 30° F. above that of the room. I have usually placed the slides over hot water pipes or on the mantelpiece.f arsenate of silver at which precipitation occurs without a nucleus. The three concentrations may be reckoned chemically. There are also three corresponding coefficients of diffusion. Let us inquire how the period dx may be expected to depend on these quantities and on the distance x from the boundary at which it occurs. Now doc, being a purely linear quantity, can involve the concentrations only as ratios; otherwise the element of mass would enter into the result uncompensated. In like manner the diffusibilities can be involved only as ratios, or the element of time would enter. And since these ratios are all pure numbers, dx must be proportional to x. In words, the linear period at any place is proportional, cceteris paribus, to the distance from the original boundary. In this argument the thickness of the film— another linear quantity—is omitted, as is probably for the most part legitimate. In imagination we may suppose the film to be infinitely thin or, if it be of finite thickness, that the diffusion takes place strictly in one dimension.