Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/232

 If the sediment remains, it is a sign that more barium hydroxide is to be added to the original solution. It is advisable, always to work with very dilute solutions of sulphuric acid and barium hydroxide, otherwise one may easily overshoot the mark.

When the solution is free from sulphuric acid and baryta, we proceed to filter it through a doubled sheet of folded filter paper, or, by means of a filter pump, through a hardened filter impregnated with animal charcoal. This process can be hastened by the use of a centrifuge. The precipitate of barium sulphate is stirred up with distilled water, well kneaded in a mortar with water, and then filtered again. It is advantageous, in order to ensure a good output of peptone, to repeat this washing out with cold water many times. The ninhydrin test can be applied at this stage, as a test of the satisfactory washing out of the precipitate. To a portion of the filtrate about 1 c.c. of ninhydrin is added, and the mixture is boiled for one minute. If the coloration is faint, or even negative, then the process of washing-out is discontinued.

In the meantime, the process of concentration has been begun. As solutions of peptones produce a great deal of scum, the apparatus represented in fig. 9 is used. The latter allows the peptone solution to evaporate to dryness, at about 40° C., under highly reduced pressure. The drop funnel serves the