power, and others again by electricity. The most satisfactory one which has been designed for hand power is that of Bausch & Lomb, of Rochester. This instrument is portable, and has the advantage of great mechanical simplicity. It is, moreover, easy to reach a high speed with it with the exercise

FIG. 35.-The Purdy electric centrifuge.

of comparatively little energy. It is also adapted for use in the examination of blood by means of the hematokrit. With water-driven centrifuges it is difficult to reach a sufficiently high speed with the water-pressure obtained in towns in this country. When they are allowed to run for a correspondingly great length of time they are very satisfactory.

The most convenient electric centrifuge is that designed by Purdy, of Chicago. This machine. is built to be run with alternating or direct current at any voltage that may be desired. It is, of course, necessary in ordering a machine to state the kind

FIG. 36.-Tubes for the Purdy centrifuge: a, percentage tube; b, sediment tube.

of current and the voltage required. A very convenient rheostat is provided, by means of which any desired speed may be obtained. There is also furnished with the later instruments a speed-counter which records the speed at which the instrument is

running. All electric centrifuges have the disadvantage that they require a certain amount of care in handling and adjustment. They also do not work well if kept in the rather trying atmosphere of the laboratory. On the other hand, they are by far the most convenient instruments for the pur

FIG. 37.—The Bausch & Lomb spiral-gear urinary centrifuge with tubes (one-fourth actual size).

pose, and in fact where many examinations of urinary sediments have to be made in the course of the day are the only machines for the purpose.

It has been proposed to use the centrifuge for the estimation of those constituents of the urine which can be separated out in the form of insoluble precipitates. To this class belong albumin, the chlorids

precipitated with silver nitrate, the phosphates with magnesia mixture, etc. This is scarcely possible except where the conditions as to speed and time of centrifuging, and temperature, are very carefully attended to. It has been found that comparatively small alterations in the speed of centrifuging have a relatively large influence in the bulk of the precipitate. Hence this method can at its

best yield a very rough approximation of the real amount of the substance precipitated. As a clinical method for comparative work, however, it is not without value.

The Inorganic Sediments of the Urine.Those inorganic sediments which occur in the urine are: The phosphates of ammonium, sodium, calcium, and magnesium, phosphates of calcium and

magnesium, and calcium carbonate.

These sediments occur only in alkaline urine, while in acid urine the sulfate of calcium is rarely found.

The common organic constituents of urine sediments are: Uric acid, sodium and ammonium urates, calcium oxalate, and more rarely hippuric acid, cystin, tyrosin, and leucin. In alkaline urine ammonium urate is not an uncommon sediment. By

FIG. 39.-Triple-phosphate crystals (Ogden).

far the most commonly occurring sediment in alkaline urine is the triple phosphate of sodium, ammonium, and magnesium. This frequently makes up the major part of the stone or calculus following a cystitis.

The phosphates occur both in the amorphous and crystalline conditions. The crystalline forms of the phosphates are again subdivided into two forms, depending on the way in which the crystals separate