The blue drawing paper of commerce, which is frequently employed for technical drawings, is not very durable. For the production of a serviceable and strong drawing paper, the following process is recommended. Mix a solution of

Gum arabic......... 2 parts

Ammonia iron citrate. 3 parts

Tartaric acid........ 2 parts

Distilled water....... 20 parts

After still adding 4 parts of solution of ammonia with a solution of

Potassium ferricyanide 2.5 parts

Distilled water......10.0 parts

allow the mixture to stand in the dark half an hour. Apply the preparation on the paper by means of a soft brush, in artificial light, and dry in the dark. Next, expose the paper to light until it appears dark violet, place in water for 10 seconds, air a short time, wash with water, and finally dip in a solution of

Eau de javelle..... 50 parts

Distilled water.....1,000 parts

until it turns dark blue.

Filter Paper

This process consists in dipping the paper in nitric acid of 1.433 specific gravity, subsequently washing it well and drying it. The paper thereby acquires advantageous qualities. It shrinks a little and loses in weight, while on burning only a small quantity of ash remains. It possesses no traces of nitrogen and does not in any way attack the liquid to be filtered. Withal, this paper remains perfectly pervious for the most varying liquids, and its filtering capacity is in no wise impaired. It is difficult to tear, and still elastic and flexible like linen. It clings completely to the funnel. In general it may be said that the strength of the filtering paper thus treated increases 100 per cent.