216 Mr. Waldie's investigations connected [No. 3? 



properties, especially with reference to its action on the human sj'stem. 

 Of late more attention has been paid to this subject, previously little 

 thought of. 



The first point requiring attention is to ascertain its quantity as 

 correctly as practicable. The plan formerly followed was to dry the 

 solid contents of the water obtained by evaporation carefully at a 

 certain fixed temperature such as 212° or 250° F., or even about 300°, 

 till the weight remained constant ; then to burn off the organic matter 

 by as moderate a heat as possible and weigh again : the loss of weight 

 was considered organic matter. But this method is liable to great 

 error, and may give grossly erroneous results. Other substances may 

 be volatilised : salts of ammonia have been mentioned, but they may be 

 included amongst organic matter ; nitrates may be partially or wholly 

 decomposed, but they generally exist in very small quantity. Earthy 

 carbonates may lose carbonic acid : — carbonate of lime will not readily 

 lose it if the heat be moderate, but carbonate of magnesia will very 

 readily, and moreover chloride of magnesium (or muriate of magnesia) 

 loses part of its acid easily. It is the magnesium salts which are the 

 chief source of loss, but this can be prevented or remedied. If the 

 contents of the water be not naturally sufficiently alkaline, a sufficient 

 quantity of accurately weighed and perfectly dry carbonate of soda is 

 added to the water on evaporating it ; the soda combines with hydro- 

 chloric acid to form chloride of sodium and water, while the magnesia 

 remains as carbonate ; and by this means, as the chlorine is not 

 separated by ignition from the sodium, the loss of chlorine is avoided. 

 The only loss is of carbonic acid, which can be restored again. This 

 is done by adding to the ignited residue in the platinum crucible 

 distilled water charged with carbonic acid and evaporating to dryness 

 by gentle heat, drying again at the same temperature as was employed 

 at the first weighing before ignition till the weight is again constant. 

 The loss of carbonic acid is by this means corrected, the acid being 

 restored, and the difference of weight shows the quantity of organic 

 matter, at least more correctly than by any other method known. 



This plan is attributed to Dr. Thomas Clark, the inventor of the soap 

 test, by Dr. W. Allen Miller in a paper* to which I shall have further 

 occasion to refer. It is tedious and troublesome, requires a fine balance, 

 * Journal of the Chemical Society for May, 1865. 



