280 R. Bunsen — Calorimetric Investigations. 



a lowering of the melting point of ice, whicli are particularly 

 deservino- of attention in the employment of the ice-calorimeter, 

 If the pnre snow which surrounds the instrument be saturated 

 with as much boiled-out or distilled water of 0° C, previously 

 shaken with air, as it can retain after the excess has been 

 allowed to drain off, there ensues, at least during the first 

 twelve hours, during which alone the observations were earned 

 on, no freezing but such a melting of the ice in the interior of 

 the instrument that it would, under these circumstances, be 

 entirely unserviceable for observation. On the other hand the 

 slightest impuritv of the snow determines such a large contmual 

 deposition of ice on the ice cylinder, that the mercury thread 

 often advances in one minute several divisions on the scale. 

 Snow which has absorbed even traces of salts from the soil, or 

 of animal, or vegetable impurities from the street pavement, 

 shows this lowering of the melting point m the most strikmg 

 manner. Eiver ice of such purity that the water obtamed from 

 it by meltino-, showed with barium chloride and silver solu- 

 tion only after a long time, a scarcely perceptible turbid- 

 ness produced in the instrument during three days two 

 grams of ice. Afler these observations it is self-apparent tbat 

 only the purest snow can be used in the experiments, it is 

 also advantageous to work in an apartment whose temperature 

 is not too high above 0°C., and not to begin the obseru- 

 tions, until the formation of ice on the ice-cylmder does w^ 

 exceed a few scale divisions in the hour. Above_ all, however, 

 care must be taken that, before the instrument is allowea w 

 take, in the snow, a constant temperature, a small layer oi 

 water has been formed by melting between the glass wails ana 

 the adjacent ice-cylinder, in order that unequal tension ana 

 consequent elastic after-effect may be avoided. 



The accuracy of the observations depends, no^J^'®^;. • li 

 essentially upon the. care which has been taken m tbe ^f^f ^^^ 

 arrangement of the instrument, that all the air absorbea uv 

 mercury and water be removed. This is to be attamed m t^^ 

 following manner : the instrument, filled to the half ^itji do^ 

 out water, is fastened with the mouth turned downward on_ 

 >holder, the mouth of the tube c, fag. 



which ii 



not yet provided with an iron casing, is sunk m »>"-- 

 kept in constant ebullition in a beaker-glass, and the 

 the instrument which reaches to fi kept boiling untu 



evaporated to i If now the lamp used to heat the app ., 

 be removed, the latter will fill itself with airless jater. ^^ 

 allowed to cool and is then, after it has been placed W'f^^ 

 in the drawing, filled to the height (i with so «iu«^ ' ;i 'h 

 boiled-out mercury, that the mercury level m t^®, ' ^,^^ter 

 stands about equally high with that in the tube c. i^ 



