ai8 PHILOSOPHICAL TRANSACTIONS^ [aNN6 1675. 



proceed with as many mixtures or degrees of allays^ as you think fit ; or aS 

 many as may be conveniently distinguished on the stem : ahvays observing, that 

 whatever be the proportions of the two ingredients, the weight of the mass in 

 the air be just the same with that of the pure gold. 



By the same method the different allays of pure silver may be examined, on 

 the mixing of any proportions of copper, or any other metal lighter in specie 

 than silver. And by the same way, with a slight variation, it will not be difficult 

 to estimate how much divers coins, whether of silver or gold, are more or less 

 debased by the known ignobler metal, mixed in the proposed piece. These 

 estimates, which may be made without much trouble, will come nearer the 

 truth ; not only than the estimates wont to be made by the touchstone, but 

 perhaps also than some of those made with trouble and charges. 



It may also be used to examine other mixtures, besides allayed coins; and if 

 the instrument be adjusted to an ounce, suppose of pure copper, it may assist 

 in making an estimate of the allay of tin, or the quantity of it added oftentimes 

 to copper, for making different sorts of bell-metal, and of those metallic specula, 

 whether plane or concave, called steel mirrors ; as also of solders, consisting of 

 certain proportions of silver and brass or copper; in all which, and divers others, 

 the discovery of the proportion of the ingredients may, on some occasions, be 

 useful to tradesmen, as well as agreeable to virtuosi. 



On the Swimming Bladders in Fishes, By Mr. Ray. N° 115, p. 349. 



I was much pleased and satisfied with the ingenious conjecture I found in 

 your Transactions of May last, N° 114, concerning the swimming bladders of 

 fishes; and persuade myself that the author has hit upon their true use, viz. to 

 sustain or keep them up in any depth of water. For 1 . It has been observed by 

 some, and I find it in Mr. Willughby's general notes of fishes, that if the 

 swimming bladder of any fish be pricked or broken, such a fish sinks presently 

 to the bottom, and can neither support nor raise itself up in the water. — 2. Flat 

 fishes, as soles, plaise, &c. which lie always grovehng at the bottom, have no 

 swimming bladders that I could ever find. — 3. In most fishes there is a manifest 

 channel leading from the gullet or upper orifice of the stomach to the said blad- 

 der, which doubtless serves for conveying air into it, as may easily be tried by 

 any one that pleases. But though air may be received into the bladder, yet is 

 there a valve or some other contrivance to hinder the egress of it; for you shall 

 sooner break the bladder than force any air out by this channel. Yet in 

 sturgeons Mr. Willughby has observed, that pressing the bladder, the stomach 

 presently swelled : so that it seems in that fish the air passes freely both ways. 



