VOL. v.] PHILOSOPHICAL TRANSACTIONS. 54g 



After the author has given us these experiments, he undertakes to determine 

 the quantity of the refractions in tliis double refracting body; and he finds, after 

 several trials, the angle of inclination to the angle of refraction in this Iceland 

 crystal, to be as 5 to 3. 



An Extract of a Letter from a French Gentleman, concerning a Way 

 of making Sea-water siveet* N" 6*7, p- 2048. 



Mons. Hauton's secret of making sea water sweet, consists first in a precipi- 

 tation, made with oil of tartar, which he knows to draw with small charges. 



* As a knowledge of the means whereby fresh or sweet water may be procured from salt water is 

 of the utmost importance to seafaring men, we shall here offer some further remarks on this subject. 

 Sweet water may be obtained from salt water by two methods, by freezing such water or by dis- 

 tilling it. 



When sea water is exposed to a degree of cold somewhat below the point at which fresh water 

 freezes, its power of holding muriate of soda and other saline substances in solution, is in part de- 

 stroyed; ice is formed on the upper surface, while the fluid portion underneath becomes a concentrated 

 brine. This ice when melted yields a water, which contains so little saline matter as scarcely to be 

 distinguished from fresh water by the taste, or indeed by chemical tests. It is evident, however, that 

 this method can only be resorted to in certain latitudes or at certain seasons of the year. 



The other method, therefore, viz. that of distillation, is greatly to be preferred, being feasible (with 

 a proper apparatus) at all times and in all situations, and, when properly conducted, yielding a water 

 as pure and as sweet as that procured by congelation. It was formerly supposed that in order to ob- 

 tain fresh water from sea water it was necessary to add to this last, before the distillation, calcareous 

 earth, potash, or certain other substances, for the purpose of absorbing and retaining a bituminous mat- 

 ter, which all sea water was supposed to contain in greater or less quantity, and to which was ascribed 

 the unpleasant empyreumatic taste of the water distilled from it, especially if too strong a fire is em- 

 ployed, or tlie distillation is pushed too far. Dr. James Lind, however, has proved that such addi- 

 tions are useless, since pure rain water contracts in like manner a bvunt taste by distillation; which 

 shows that it is derived from the action of the elementary water on the heated metallic vessels. This 

 disagreeable flavour, however, goes off, for the most part, on exposing tlie distilled water to the 

 air. Nothing more is requisite, then, for obtaining fresh water from salt water, than to be provided 

 with a common still ; or with still-head covers made to fit the coppers used for boiling provisions on 

 board of ship; and a worm-tub or cooler for condensing the steam, (See Dr. Lind's Essay on pre- 

 serving the health of Seamen. Also, the Appendix to his Essay on Diseases incidental to Europeans 

 in Hot Climates.) Some years after tliis discover)' was made known by Dr. Lind, [It would appear 

 however that the simple distillation of sea water, for the purpose of procuring fresh water, was 

 practised by Sir Richard Hawkins, in the reign of Queen Elizabeth. See the Bishop of LlandafTs 

 (Dr. Watson's) Chemical Essays, vol. ii.] an improvement was suggested by Dr. Irving, in the 

 mode of distillation; wherein he substituted, for the condensation of the steam, a large open pipe 

 kept constantly wet with mops, in place of the small slender pipe passed through a tub of cold water, 

 in the usual way. This, from being applied to larger coppers than the common method ever had been 

 in the distillation of sea water, yielded in a given time, and with the same quantity of fuel, a larger 

 quantity of fresh water. 



