The analysis of food and drugs / by T.H. Pearmain and C.G. Moor.

  • Thomas Hames Pearmain
Date:
1897-1899
Catalogue details

Licence: Public Domain Mark

Credit: The analysis of food and drugs / by T.H. Pearmain and C.G. Moor. Source: Wellcome Collection.

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    Each of these methods of examination of water has its own special importance, but it may be as well to indicate the information which is gained by these methods of examination. Undoubtedly the chemical examination is usually the most important. It permits a far larger number of independent factors to be accurately estimated; and although the consumption of the substances discovered may in itself be indifferent to health, the chance of detecting variation in composition, which is the most important sign of danger in waters which may sometimes or usually be safe, is considerably greater than by other methods. Its least merit is the certainty by which directly poisonous, as distinguished from infectious, material is detected ; for such material is of rare occurrence. In the same way, but not for the same reason, it may be said that the least merit of bacteriological examination is the direct detection of specific infective organisms. Where such organisms are detected the supply is, of course, to be condemned. But it must be remembered that infective organisms are not dissolved in the water and uniformly distributed over it. They are solid bodies floating in it, and as only a very small sample of the total supply is examined, they may be present in the supply and absent from the particular samples chosen. They are also in all cases accompanied by other organisms, some much better capable of growing in water; and when they reach the analyst, they may be so far attenuated, that they fail to develop in his media, and though present, may escape recognition. The chief merits of bacteriological examination are two. On the one hand, it furnishes a factor capable of quantitative estima- tion, by which variations in the composition of the supply may be detected. On the other hand, it enables organisms which, though themselves indifferent, are usually intro-
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