216 Mr, Waldie’s investigations connected [No. 3, 
properties, especially with reference to its action on the human system. 
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. Harthy 
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. Ii 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 organie 
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 ina 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. 
