28 PROCEEDINGS OP THE AMERICAN ACADEMY 



The paper, cut into small pieces, is placed in a glazed porcelain 

 dish and moistened with 1 to 5 c. c, according to the amount of 

 paper taken, of strong sulphuric acid (sp. gr. 1.8) to which has been 

 added about one thirtieth of its volume of strong nitric acid (Appen- 

 dix, 2. e). The paper and acid are stirred with a thick glass rod 

 until the paper has absorbed the acid, and the dish is then placed on 

 a ring and heated by a low flame, the mixture being stirred continu- 

 ally, until the paper is thoroughly charred (App., 1). This may be 

 determined by the dry " crumbly " appearance, and by the amount 

 of fuming, it being necessary to heat until the nitric acid is expelled. 

 Usually, the first heating is enough, as, even if a trace of nitric acid 

 is left, it does no harm (App., 2./) ; yet, if one has reason to think 

 that considerable nitric acid is held back, it is best, after cooling, to 

 add a few cubic centimeters of water, and heat again until the fumes 

 of sulphuric acid appear. On cooling, the " char " is moistened with 

 a few drops of water, and then about 5 c. c. of water are added. The 

 mass is triturated with the thick rod until all lumps are thoroughly 

 broken up, heated to boiling to expel sulphur dioxide (App., 5. d) and 

 filtered hot (App., 3). 



Filtration of the Extract. — In filtering, time is saved and greater 

 accuracy assured by using a filter pump, and for filtration I have 

 found most convenient a sideneck test tube of 25 to 30 c. c. capacity. 

 This is fitted with a rubber cork through which passes a small funnel, 

 the end reaching just below the side tubulus of the test tube. With 

 the tubulus is connected the pump, and in order to prevent accidental 

 contamination of the tubulus, (through which the extract is afterwards 

 poured into the Marsh apparatus,) I use a connecting glass tube, 

 which may be rinsed before and after each filtration. This tube is a 

 small U tube not over 25 cm. in total length, with two bulbs blown 

 in the lower part of the U. The filter paper is of small diameter, 

 not over 8 cm. and the lot should be tested for arsenic previous to 

 cutting the round filters. As a strengthening cone at the apex will 

 be found convenient for this, as well as for other filtrations, a square 

 piece of cheese cloth laid under the paper and folded with it. 



After filtering, the char is washed with small quantities of hot 

 water until the filtrate and washings fill the test tube. The extract, 

 after cooling, is ready for weighing or measuring, and introduction into 

 the apparatus. 



The Apparatus. — Two points will have been noticed in working 

 with the ordinary form of generating flask : the time required for dis- 

 placement of the air, and the impossibility of governing the evolution 



