VOL. LXIX.] PHILOSOPHICAL TRANSACTIONS. 563 



adjusted to distinct vision by means of an adjusting screw, which moves the 

 whole eye tube with the micrometer nearer to or farther from the object-glass, as 

 telescopes are generally made; or the same effect may be produced in a better 

 manner, without moving the micrometer, by sliding the part of the eye tube m 

 on the part n, by help of a screw or pinion. The micrometer is made to take 

 off occasionally from the eye tube, that the telescope may be used without it. 



XXVllI. Of the Airs extracted from different Kinds of Waters ; with Thoughts 

 on the Salubrity of Air at different Places. By the Abbe Fontana. p. 432. 



The following experiments were made at Paris in 1777 and 1778 on the air 

 extracted from various kinds of waters. 



Mr. F. extracted the air from the water of a well by means of common fire. 

 The water was then made to boil in a large matrass of tin, which had a long 

 tube of the same metal, which being bent into 2 different directions was with its 

 extremity immerged in a tub of cold water. The matrass and its tube were 

 entirely filled with water: the air which came out of it was received into 3 dif- 

 ferent vessels. The air of the 1st vessel, by being shaken in water, was 

 diminished a little; the air of the 2d was diminished of half its bulk, or rather 

 more; and the air of the 3d vessel was diminished exceedingly. The residuums 

 of air that remained imabsorbed were more or less phlogisticated. 



Another time Mr. F. obtained almost entirely fixed air, excepting a httle which 

 remained unabsorbed by water, and was partly phlogisticated. A 3d time the 

 air of the well water was made to pass through mercury into a tube anointed 

 with oil of tartar, and it occasioned a crystallization just like that which the 

 purest fixed air is used to do. A 4th time he impregnated with this air a quan- 

 tity of common water, which absorbed its own bulk of it, and became by these 

 means acidulous, exactly like water with the purest fixed air. This water turned 

 the tincture of turnsole red, and precipitated the lime in lime water. Another 

 time a light was successively extinguished, and a bird died instantly in this air. 



The water of the river Seine, filtrated through sand, as it is drank at Paris, 

 was treated in the same manner as the well water. The air extracted from that 

 water was J absorbed by water, when shaken in it; the remainder, when treated 

 with the test of nitrous air, gave ii — 4, ii + 1 ;* when the common atmos- 

 pheric air treated with the same nitrous air gave ii — 4, 11 + 8. It was there- 

 fore sensibly better than the atmospheric air, which during 3 years of experiments 

 made at Paris, he constantly found to be inferior to the air of the Seine water, 

 extracted as above. Having repeated the experiment, he received the air into 2 

 different receivers. The 1st of which, by being shaken in water, was dimi- 



* See p. 529 of this Abridgement, for an explanation of this measure. 



Ac I 



