1 76 INTRODUCTION TO GENERAL PHYSIOLOGY 



Add acid until it becomes red, and note that the extra quantity 

 required is distinct. This indicator does not show a solution to be 

 acid until its hydrogen-ion concentration has risen to o.oi (io~ 2 ) 

 normal. The colour is orange-red at o.ooi normal. The difference 

 in the amount of acid required will, of course, be more obvious if 

 o.oi molar acid is taken instead of the o.i molar. 



As exercises in the use of indicators, determine the hydrogen- 

 ion concentration of urine, which will usually be found to be acid 

 to methyl-red, just alkaline to methyl-orange. The colour of the 

 latter dye in it is brownish, not yellow, corresponding to a 

 hydrogen-ion concentration of io~ 4 (see P., p. 189). Blood serum 

 after exposure to the air is yellow to neutral-red ( = io~ 9 ). Brought 

 into equilibrium with the last fraction of a deep expiration 

 ( = alveolar air, containing 4.5 per cent, of carbon dioxide) it becomes 

 orange-red. That is, just on the alkaline side of neutrality. 

 Neutral-red in pure distilled water is red, not crimson. The 

 slighest trace of acid turns it crimson ; of alkali, yellow. It is thus 

 a valuable indicator in the region about the neutral point (io~ ri ). 

 The saturation of liquids with gas mixtures, such as alveolar air, is 

 performed in vessels containing a large volume of the gas in pro- 

 portion to the liquid. For the above purpose, a stoppered bottle 

 of about 100 c.c. capacity will serve. Two or three c.c. of the 

 serum with a drop of o.i per cent, neutral-red are placed in the 

 bottle and the expired air breathed into the bottle through a glass 

 tube. The stopper is replaced quickly, and the bottle rotated so 

 as to make a thin layer of the serum over the surface. 



The Colloidal State 



Colloidal Gold is readily prepared by Faraday's method as 

 follows : Take a solution of gold chloride in pure distilled water, 

 containing about one part of the salt in 8,000 of water. Put it in 

 a clean bottle or flask. Add a drop of a solution of phosphorus in 

 carbon bisulphide. Shake and leave for a few hours. A beautiful 

 clear red solution is obtained. 



This solution is shown to contain solid particles in suspension 

 by passing a bright beam of light through it (Faraday phenomenon). 

 The beam from an arc-lamp, brought to a focus by a condenser, is 

 appropriate. To avoid disturbing reflections at the surfaces of the 

 glass, the vessel may be immersed in water in a large beaker. 

 This fact was described by Faraday, and correctly interpreted as 

 showing the presence of particles of metallic gold. Tyndall pointed 

 out subsequently that if the track of the beam be looked at through 

 a Nicol prism, it is found to be polarised, being extinguished in a 

 particular position of the prism. This shows that the size of the 

 particles is near that of the wave length of light. 



