388 PROCEEDINGS OF THE AMERICAN ACADEMY. 



seemed the best suited to the present purpose. The filtered sokition was 

 therefore titrated with a solution of approximately hundredth uormr,l 

 permanganate, of which one cubic centimeter corresponded to 0.00062 

 gram of calcic oxalate. The amount of the solution, 0.12 c.c, which 

 was required to impart the usual pale pink color to the assay, was 

 always subtracted from the total volume run out. In each analysis ex- 

 actly a decilitre of calcic oxalate solution was used. 



These results are typical of a number of similar determinations which 

 were made in this way. The first portions running through a filter 

 paper always seemed to contain slightly more calcic oxalate than the 

 later portions, probably owing to the escape of fine particles which are 

 retained by the filter when it has become somewhat clogged. The later 

 values are probably the more reliable, hence it is clear that hot water 

 will dissolve easily over a centigram per litre of calcic oxalate, while 

 water at the ordinary temperature dissolves nearly seven milligrams 

 per litre. 



This solubility is altogether too great to be passed without heed in 

 precise work. Its magnitude was such as to make desirable a more 

 exact determination of a more nearly saturated solution. 



The first problem to be solved in this connection was the retention of 

 even the finest particles. The best filter papers of Schleicher and 

 Schiill, Dreverhoff, and others were tested, all with the result already 

 described ; namely, that the first filtrate always yielded slightly more 

 calcic oxalate than the following ones. Finally it was decided to use 

 four layers of paper, and to reject at least half a litre of filtrate before 

 beginning to collect for analysis. The filtration was effected by means 

 of a platinum inverted filter devised by J. P. Cooke,* and a simple 

 arrangement of tubes and stopcocks made the rejection of the first and 

 the collection of the subsequent portions of filtrate an easy matter. The 

 filtered liquid appeared perfectly clear on inspection in strong light. 

 The inverted filter made it possible to maintain the filtering liquid at 

 the desired temperature. The time of digestion at the higher tempera- 

 tures was an hour; but at 25° an hour and a half was allowed. A 

 very large platinum dish served as the vessel for digestion, and the liquid 

 was suitably protected from the products of combustion of illuminating 

 gas and other impurities. A diagram of the apparatus will furnish all 

 further necessary explanation. As before, a hundred cubic centimeters 

 of the solution served for each titration. 



* These Proceedings, 12, 124 (1870). 



