NOTES AND ABSTRACTS IN CHEMISTRY AND PHARMACY. 487 
hot porcelain tube, and finally to pass through a Liebig’s bulb containing a 
solution of nitrate of silver. The vapour of the chloroform diffuses into the 
stream of air, is decomposed in the heated tube, aud the resulting chlorine pre¬ 
cipitates the silver solution.* 
The ’Washing' of Precipitates. 
The January number of the ‘ Philosophical Magazine ’ contains a full trans¬ 
lation of a memoir published in the Annalen der Chem. und Pharm., by 
R. Bunsen, on the washing of precipitates. This paper is a very valuable and 
interesting oue. The author deduces a mathematical formula for calculating 
the amount of water necessary for washing a precipitate, aud describes a new 
form of vacuum filter for hastening the process of filtration in analytical 
operations. 
A precipitate is washed by repeatedly diffusing it through a given quantity 
of water, the portion of liquid not mechanically retained being each time sepa¬ 
rated from the precipitate either by decantation or by draining on a filter. 
This process is continued until the amount of impurity becomes so minute that 
its presence may be disregarded. Precipitates obtained in the course of chemi¬ 
cal analysis may in all cases be assumed to be sufficiently washed when the im¬ 
purity retained by them amounts to no more than the T ooVoo part. 
Since some of the principal sources of error in analytical work consist in the 
incomplete or in the too protracted washing of precipitates, it becomes im¬ 
portant to know how to ascertain the progress of the washing throughout the 
several stages of the process. By employing the same volume of water at each 
successive addition, and estimating its relation to that of the precipitate remain¬ 
ing at the bottom of the vessel or upon the filter, we can find from the follow¬ 
ing table the number of times it is necessary to decant in order to diminish the 
amount of impurity in the precipitate to the ^ 5 , ^00, M> or toItoo P a r fc - 
Column I. shows the relation between the volume of the precipitate aud that of 
the washing-water employed for each successive decantation ; column II. the 
number of decantations required to diminish the amount of impurity to the 
necessary extent; and column 111 . the total volume of water obtained from the 
several decantations. 
When the washing process is performed in a beaker, the relation between the 
volume of the precipitate aud that of the liquid may be easily determined by 
holding a strip of paper along the side of the vessel, and marking upon it the 
respective heights of the precipitate and supernatant liquid; then, on folding 
the portion of paper lying between the two marks in such a manner that each 
fold corresponds to the height occupied by the precipitate, the number of folds 
will give the number in column I. to find in column II. the number of decanta¬ 
tions needed to wash to the required extent. If the washing be conducted as 
in the ordinary method of filtration, the capacity of the filter to be employed is 
first learnt by filling it with water from a burette or measure-glass, the volume 
it holds being represented by V. The precipitate is next collected on the 
filter, and allowed to drain; then the filter is filled up from a graduated wash 
ing bottle, mixing the precipitate as thoroughly as possible with the water. 
The volume of water required for this, deducted from the volume V, gives the 
volume of the precipitate, or v. When V is divided by y, the product expresses 
the relation between the volume of the precipitate and that of the washing- 
water. This number is sought in column I., and opposite to it in II. is the 
number of times it is necessary to refill the filter in order to wash the precipi¬ 
tate to the required extent. 
* Extracted from the Journ. de Phann. d’Anvers. 
