"_The calculus composed of a mixture of the phosphate of lime and triple phosphate of magnesia-and-ammonia, or the fusible calculus_, is commonly whiter and more friable than any other species, resembling sometimes a mass of chalk, and leaving a white dust on the fingers. This species is generally not laminated. Occasionally, however, it separates readily into laminae, the interstices of which are often studded with sparkling crystals of the triple phosphate. The variety of this species which is not laminated often acquires a very large size, and assumes the form of a spongy friable whitish mass, evidently moulded to the contracted cavity of the bladder in which it has been formed. This species of calculus occurs very frequently.--_Chemical characters._ It may be readily distinguished by the ease with which it melts before the blow-pipe. It also dissolves readily in acids, and particularly in dilute muriatic acid; and if to the solution, oxalate of ammonia be added, the lime is precipitated alone, and the magnesium may be afterwards separated by the addition of pure ammonia. "_The alternating calculus_, as the name imports, may consist of different layers of any of the preceding species. Hence its general appearance, texture, &c. will depend entirely on the composition, and may be very varied. Most commonly it is composed of a lithic acid or mulberry nucleus, and an external crust of the fusible calculus. In some rare instances, it is composed of laminae of all three of these substances, and sometimes of even more--the mixed phosphates still continuing to constitute the external crust. This species of calculus often acquires a very large size and is very common.--_Chemical characters._ The chemical characters must of course vary with the composition; and as the different substances of which it is composed must almost certainly be some of the preceding, the nature of the different laminae can be readily ascertained by what has been already stated," p. 79, et seq. In the chapter on the comparative prevalency of different forms of urinary deposite; and the order of their succession, we have a number of important facts and observations. Dr. PROUT calculates, from the data collected by him, that about one-third of the urinary calculi which occur, are of the lithic acid species, and that another third are formed on a nucleus of this acid. Hence, "we may assert," says he, "that at least _two-thirds_ of the whole number of calculi originate from lithic acid; that is to say, if a lithic acid nucleus had not been formed and detained in the bladder, two persons at least out of three, who suffer from calculus, would have never been troubled with that affection. This is a most important fact, and deserves to be constantly borne in mind."