142 ANNUAL OF SCIENTIFIC DISCOVERY. 



length of this wire is proportional to the increase of temperature 

 S ; it admits of measurement. 



" The same apparatus is applicable to vapors. The liquid to be 

 examined is distilled as regularly as possible ; the current of 

 vapor is at first superheated by the first bundle of wires ; it after- 

 wards traverses the second, becomes heated by a quantity 0, 

 which is measured as before ; the vapor is condensed, and after- 

 wards weighed. In order to take into account the irregularities 

 of the distillation, it is necessary to observe the apparatus from 

 minute to minute. 



"III. Latent Heat. In order to measure latent heats, a 

 double alembic is employed, of which one part is exterior ; the 

 liquid in it is caused to boil, and the vapor is brought there after 

 having been condensed by a refrigerator ; the effect of this is sim- 

 ply to raise to the boiling temperature the interior alembic, 

 which contains the same liquid, and in which is immersed the 

 spiral, the resistance of which is known for every temperature. 

 The vapor which forms in the second apparatus is collected dur- 

 ing 10 minutes before the passing of the current; there is scarcely 

 any ; the circuit is then closed which determines a rapid boiling. 

 The heat supplied is known ; the vapor which it has formed with- 

 out change of temperature is weighed, and the latent heat is 

 deduced. 



" IV. TJie two Specific Heats. A third application of the same 

 principle can be made. In a large bell-glass filled with air, a 

 metal wire is stretched ; an intense current is passed for a short 

 time through it. which develops a determined quantity of heat ; 

 a fraction of this disappears by radiation ; the remainder, which 

 is constant, gives heat to the gas, which can be measured in two 

 ways, either by increase of the volume at constant pressure, or 

 by increase of the pressure at constant volume. From these two 

 effects the ratio of the two specific heats can easily be determined, 

 and the number found is about 1.42, a number indicated by the 

 velocity of sound." Comptes Rendus, trans, in Phil. Mag. 



SUDDEN BREAKING-UP OF ICE. 



A letter from Canada, in " Nature," for June 23, gives some 

 curious facts regarding the sudden breaking-lip of ice coverin^ 



^j <-J ^j i. ^5 



lakes and rivers. " The ice on our inland lakes is generally 2 or 

 3 feet thick. As the spring advances, an inch or two may be 

 melted away from the lower surface, and somewhat more from 

 the upper one, but the thickness is not materially reduced until 

 its final disappearance. The first sign of the approaching break- 

 up is that the ice becomes dry, from the prismatic structure hav- 

 ing commenced to show itself, allowing the surface water to 

 percolate through the interstices; it is then said to be honey- 

 combed. In this state the lower layers of transparent ice are still 

 solid, though if you cut out a block the prismatic structure is very 

 evident; but the upper portion, which has been formed from a 

 mixture of snow and water, readily breaks up under your feet in- 



