38 



Methods of estimating accurately 



quantity of the fine precipitate either adheres to its surface inse- 

 parably, or even sinks into its substance. The best is a sort of 

 thinish blotting paper, with one of its surfaces apparently glazed, 

 from which the dried precipitate easily scales off, leaving but a very 

 small trace upon it. Weighing the precipitate, on a previously 

 weighed filter, is inadmissible, as the paper will vary in weight 

 according to its degree of exsiccation, and it is scarcely possible to hit 

 the precise stage of dryness in which it was when first weighed. 

 Burning the filter and adding the ashes to the precipitate will give 

 an excess of weight, unless a corresponding portion of the paper be 

 burnt, and the weight of its ashes subtracted. Even in this case, 

 there is a source of error in the obstinate retention, in spite of all 

 washing, of some saline matter in the very edge of the filter, a source 

 of error which also holds in regard to the estimation of substances 

 upon a filter previously weighed. 



All precipitates to be weighed should, immediately previous, be 

 exposed to a heat visibly red, with the exception of such as would be 

 volatilized or decomposed by it. These are usually dried, for an 

 hour or two, at the heat of boiling water. The ignition of small 

 quantities is best performed on platina over a spirit lamp. 



The first step in the process is to ascertain the substances pre- 

 sent in the water, by testing. All natural waters contain a little 

 atmospheric air, sometimes with an excess, sometimes with a 

 deficiency of oxygen. Unless the analysis be a scientific one, this 

 may be neglected. Carbonic acid gas is a usual ingredient. This is 

 most certainly detected, when free, by boiling the water in a retort or 

 flask fitted with a bent tube, and receiving the evolved gas in lime or 

 barytes water, when white carbonate of these earths will be preci- 

 pitated. Some of the usual methods of preventing error from the 

 absorption of the carbonic acid gas in the atmosphere should be 

 adopted. Carbonates are precipitated by boiling, and recognised by 

 effervescing with acids. Sulphuretted hydrogen is known by its smell 

 or when in minute quantity, by the blackening of the recent water 

 by salts of lead, bismuth, or copper. It is soon decomposed on 

 exposure to the air. Sulphuric acid is indicated when a white 

 precipitate falls on adding to the water first nitric acid to prevent the 

 precipitation of a carbonate, and then nitrate of barytes. The liquid 



