1893.] MICROSCOPICAL JOURNAL. 133 



quality of the water, as regards present or actual pollution, and 

 the less likely is the water to contain any organic matters of ani- 

 mal origin." The fact, however, that waters which are entirely 

 uncontaminated by animal matter may still contain a considera- 

 ble quantity of nitrogen derived from vegetable matter, either 

 growing in the water, or with which it has been in contact, is 

 referred to, and, in Dr. Frankland's view, satisfactory methods of 

 interpretation sufficient for every possible case pointed out. 



The most interesting featui"e of Dr. Frankland's system, as well 

 as the most important in the present connection, is the method by 

 which he determines whether or not a given water has been pre- 

 viously contaminated by sewage or other animal contamination. 

 In making this determination, the following circumstances 

 govern. In the first place, throwing out of consideration the free 

 nitrogen of the dissolved atmospheric air, we may say that the 

 element nitrogen exists in water either as a constituent of ammonia, 

 organic matter, or in combination with mineral substances as 

 nitrites and nitrates. In these several forms the nitrogen con- 

 stitutes a record of not only the present pollution but of that which 

 is past ; thus the organic nitrogen furnishes the record of that 

 portion of the pollution which is a matter of the immediate pres- 

 ent, the ammonia a record of that which is past, but so recently 

 as to still constitute an element of danger, while the nitrites and 

 nitrates indicate the pollution which formerly existed but which 

 probably has been rendered innocuous through the action of the 

 forces producing nitrification. In regard to these forces it may 

 be noted that their action is comparatively rapid in the pores of 

 an open soil, while in the water of rivers and lakes they act much 

 more slowly. The nitrites and nitrates, when once formed, how- 

 ever, mark the complete reduction of the nitrogen to the mineral 

 state and, remaining dissolved in the water, constitute " a record 

 of the sewage or other analogous contamination to which it has 

 been subjected since its descent to the earth as rain." In order to 

 obtain a concrete expression for the amount of the previous con- 

 tamination, the amount of total combined nitrogen contained in 

 solution in 100,000 parts of average London sewage was taken 

 as a standard of comparison. Various analyses show from 7.06 

 to 8.363 parts of total combined nitrogen in roo,ooo, but for sim- 

 plicity a round number — 10 — was assumed as the total amountof 

 combined nitrogen in solution in 100,000 parts of average London 

 sewage. Inasmuch as this number is considerably higher than 

 that furnished by actual analysis, it is clear that the assumed 

 standard is only a conventional one ; hence the indications de- 

 rived from it, while comparable among themselves, are still only 

 comparable with London sewage by the use of a correction 

 factor, although as used by Dr. Frankland such a correction has 

 not been applied, chiefly because of the essentially varying com- 

 position of the sewage itself; from hence it results that a compar- 

 ison made at one time would only be relatively true at another. 



