LABORATORY WORK 167 



better by the dark ground method. The object of using distilled 

 water is to avoid the aggregating effect of the lime salts in tap 

 water (see under "colloids" below). 



To see the fixation of the particles by a gel, rub the gamboge 

 in warm 5 per cent, gelatin, place a drop on a warm slide, cover 

 and watch while setting takes place. The movement will gradually 

 disappear. 



Surface Tension. Soap solution is made with pure sodium 

 oleate. Make a 2.5 per cent, solution in distilled water in the cold. 

 It may take a day or so to dissolve. Add one-third of its volume 

 of strong glycerine, and after shaking, allow to stand for a week in 

 a stoppered bottle in a dark place. Then remove the clear solution 

 from underneath the scum by means of a siphon. Add a drop of 

 strong ammonia to each 200 c.c. and keep in a stoppered bottle in 

 a dark place. Do not return any that has been used to the stock 

 bottle. 



In the preparation of this and other solutions required later, a 

 chemical balance weighing 50 or 100 gm. and sensitive to i mg. 

 will be required. It is convenient to replace the left-hand scale pan 

 by a flat-bottomed porcelain basin. This is carefuly counterpoised 

 by a piece of lead suspended on the hook at the top of the opposite 

 scale suspension. A porcelain pan can be washed, so that it is 

 unnecessary to use paper for the weighing of chemicals, which must 

 never be put into the metal scale pan. 



A useful tube for blowing bubbles is a wide glass tube, such as 

 the chimney of certain paraffin lamps. A perforated cork is fitted 

 in the narrow end, and a short piece of \-'m. tubing inserted in the 

 hole. Dipping the wider end into soap solution, a film is made 

 closing the end. This is blown out into a spherical bubble and the 

 end of the finger placed on the mouth-piece. The bubble remains 

 the same size ; but if the finger be withdrawn, surface tension causes 

 the bubble to contract, driving air out. 



Make a film on the wide end of a funnel, closing the end of the 

 stem with the finger. As soon as the finger is removed, the film 

 proceeds to rise up to the narrowest part of the funnel. Thus, 

 surface energy is doing work in raising the weight of the film. 



For the ring experiment iron wire serves well. A circle is made 

 with a projecting handle. The end of that part forming the circle 

 may be soldered to the base of the handle, or merely twisted around 

 it. A little loop of fine sewing silk is tied to a point on the ring, so 

 that the loop is suspended about the middle of the space. It saves 

 trouble if it be tied to the opposite point of the ring also, but the 

 suspensions must not pull tightly. Dipping the ring into soap 

 solution in a flat dish, a film is formed in which the loop floats. By 

 moving it about with a needle, it can be made to take any shape, 



