Mechanical Science, 425 



hydrometer is g^iven at page 371, of vol. xxii. of the former series of 

 this Journal. Since then he has simplified the application of the prin- 

 ciple. " What I would suggest," he says, '* as likely to render a 

 simple glass syphon very convenient as a hydrometer is, merely to 

 put a small hole in its upper or bent part. On immersing each 

 leg of such a syphon in a separate liquor a portion of the air 

 escapes through the hole, and allows the liquids to rise in the tubes 

 to the level of their cisterns. If we now apply the finger to the 

 hole, and raise the instrument, I need scarcely say, not wholly out 

 of the fluids, the liquors will be raised in the tubes by the i)ressure 

 of the atmosphere so as to form columns elevated to heights above 

 their respective cisterns, inversely proportional to their specific 

 gravities. For the weights of the two columns must obviously be 

 equal, each being the difference between the pressure of the atmos- 

 phere and that of the included air. 



** If the tube be very wide, the effect of capillary action may in 

 most cases be neglected: so that the one column being water, we 

 divide its length by that of the other liquid, and obtain a quotient 

 which is the specific gravity of the latter. But the effect of capil- 

 lary action may be easily obviated altogether by holding the syphon 

 at two different heights, and noting the corresponding columns. 

 We obtain the specific gravity free from capillary action by dividing 

 the difference of the columns of water by the difference of those of 

 the other fluid. The greater these differences can be made sa 

 much the better ; even using, perhaps, for the shorter columns the 

 mere capillary elevations." 



In transparent liquors we may make the one pair of columns to 

 be depressions under the surfaces of the liquids contained in glass 

 cisterns; which may be effected by immersing the instrument, with 

 the hole previously stopped, to confine air. In this case, we obtain 

 the specific gravity by dividing the sum of the elevation and depres- 

 sion of the water by the sum of those of the other fluid. 



As a hole may be apt to weaken a glass tube, especially at the 

 curved part, where it should be strongest, two straight pieces of 

 glass tube may be joined by means of a bit of bent tin tube. The 

 hole may then be more easily made, and will be less apt to weaken 

 the instrument. The legs of the syphon should be graduated or 

 divided into small equal parts : this may be very easily done by 

 merely transferring to the tubes, with the assistance of a square, the 

 divisions which are already made on any scale of small equal 

 parts. It is obvious that the legs ought to be parallel. — FhiL 

 Mas. N. S. iv. 258. 



7. "Alteration of weight in RockCrystaU by fulverisation between 

 Agates. — The following experiments are by M. Pajot Descharmes. 

 A quantity of rock crystal was several times heated red hot and 

 plunged suddenly into cold water ; it was then dried ; pulverized 

 in an agate mortar, and after being sifted, weighed one ounce 

 or 576 grains. 



