It is recognised that, in addition to those chemical elements which are known to be necessary and essential constituents of protoplasm, others have a wide distribution in living matter. Such are iron, copper, zinc, aluminium and manganese. Besides these, a number of additional elements have been recorded as occurring spasmodically in animal and plant tissues. Hitherto the spectro-scope has rarely been used as a method of detecting uncommon elements in animal tissues; yet the spectroscope supplies a valuable tool for making a wide survey of tissue contents with a rapidity of which chemical methods do not allow. The present paper is the first report on such a survey, carried out on a quantitative basis. The field to be covered is, of course, vast. As a beginning, we have restricted ourselves mainly to annelids, in which whole animals were analysed, and molluscs, the separate organs of which were dissected out and dealt with. In addition, a limited amount of other material has been studied, including human organs, insects, and a few representatives of other groups. As the work progresses it is intended to investigate the principal divisions of the animal kingdom.

The epidermis is not only a protective covering against mechanical injury, but presumably protects the body also from diffusion inwards of foreign sub stances or diffusion outwards of normal constituents of the blood. In a previous paper (Whitehouse, Hancock and Haldane, 1932) it was shown that though the epidermis is permeable to water, and the actual permeation is under physiological control and plays an important part in the regulation of body-temperature, yet salts and colloidal substances do not appear to diffuse through the epidermis, so that in this respect the epidermis seems to play the part of a semi-permeable membrane. The question of the permeability of the epidermis to various substances which are met with in mining and other operations, or are applied with a remedial object, is of much practical interest, and in connection with the treatment of mining accidents from burning, the apparent impermeability of the outer epidermis to colloidal solutions of tannic acid and ionized solutions is of special importance. It is presumably the dense stratum lucidum of the external epidermis that tends to stop diffusion.The present investigation deals with the question whether the skin is entirely impermeable to electrolytes in solution. The choice of suitable substances for the tests was necessarily limited to elements which are either not present in urine or blood, or present in only very small amounts, and which could also be detected and determined very accurately.Lithium as a kation was chosen for the first series of experiments, since it is only present in very minute amounts in human blood and urine, and a very small increase in its concentration can be detected. A spectrographic method similar to that used previously by one of us (Ramage, 1929) was used to test for the presence of lithium. A measured volume of urine was rim on to a piece of MunktelFs ashless filter paper weighing about 0*11 gm. After drying in a steam oven, the paper was made into a tight roll and burnt in an oxy-coal gas flame issuing from a fused silica burner, the light being focussed by a quartz lens on the slit of the spectrograph. The piece of filter paper absorbed 0*1 c.c. of fluid easily and, if a larger quantity was necessary for the analysis, it was on May 9, 2018 http://rspb.royalsocietypublishing.org/ Downloaded from