Cxvi. the Biochemistry of Silicic Acid Viii. the Determination of Silica

THE method employed for the determination of silica in tissues and body fluids must depend on the amount and nature of the silica present. For tissues such as the lungs of workers in dusty atmospheres, where a great deal of mineral matter may be present, the ordinary gravimetric procedures are applicable. In the case of normal tissues of low silica content a micro-gravimetric method is suitable [Morgan & King, 1932], but a colorimetric method may be preferred because of the absence of the technical difficulties inherent in the micro-gravimetric procedure. The quantities of silica normally encountered in urine and blood are so small as usually to demand a colorimetric method. The silica of lungs and lymph glands consists largely of particles of mineral dust which have gained access to the tissues through the air passages. Much of it, being derived from refractory silicate rocks, is insoluble in chemical reagents (e.g. quartz, mica). This material must be decomposed before it can be estimated. For this reason it is essential to fuse the tissue, or its ash, with sodium carbonate. The endogenous "silica of constitution" of other tissues is more easily brought into solution. But it is found most convenient to fuse these also with sodium carbonate, when there can be no question that all of the silica in the tissue is being determined. In blood and urine all the silica appears to be in a soluble state. This makes possible a direct determination through a chemical reaction leading to the production of a colour without the necessity of preliminary decomposition with sodium carbonate. Several alterations and improvements have been made in the methods of silica analysis employed in this investigation since the publication of the first paper ofthe series [King & Stantial, 1933] on the "micro-determination of silica". These are outlined in the present communication, together with directions for the application of the method to a variety of biological materials, both normal and pathological, And to mineral dusts. The description of a gravimetric procedure is likewise given. Method8 Preparation of tissue for analysis. There is a large degree of variation in the silica content of different parts of some tissues. This is notably the case with lungs, where the inhaled siliceous dust settles in largest amount in the region of the hilum, and becomes concentrated in the root glands. With these tissues it is essential to obtain as representative samples as is possible, so that the analysis may represent the composition of the whole organ; wherever possible the entire organ should be taken [cf. Collins & Dible, 1935]. Tissues like liver and muscle are much more uniform in composition, and need not be sampled so carefully. The analysis is preferably done on dried and powdered tissue rather than on fresh tissue. A block of tissue or the whole organ is trimmed free from adherent material, and is washed first with tap water and then with distilled water. It is