ELEMENTARY CHEMICAL PHYSIOLOGY 191 



metric one and a volumetric to a colorimetric one. The chances 

 of error in the comparing of colours are increased by alterations in 

 the quality of the illumination, eye strain, depth and quality of the 

 colour. The advantages are that these colorimetric methods serve 

 for the detection, very frequently, of minute amounts of the reactive 

 substance, and also of substances in which gravimetric methods are 

 well-nigh useless owing to the small yield. 



The same objection, although to a lesser degree, applies to 

 volumetric methods. Here, as a rule, a single colour indicator is 

 adhered to, but it is seldom that one compares the tint obtained 

 with a standard colour. Further variations in the amount of 

 indicator added are apt to occur, with, as a result, variations in the 

 final colour value. Indicators also vary considerably as to their 

 end points (see p. 338). Volumetric methods are however invalu- 

 able and extraordinarily close duplicate analyses can be carried out, 

 provided ordinary care and accuracy is employed in the reading of 

 pipettes, burettes, etc. 



One of the great assets in gravimetric work is that the final 

 weighing can be carried out with great accuracy and can be checked 

 if necessary. The difficulty here is loss of material due either to 

 solubility or careless handling during filtration. 



As regards the statement of results the number of digits given 

 should just be sufficient to do justice to the experiment. Although 

 it would be a matter of little or no importance in an average meta- 

 bolic experiment whether the total nitrogen output was stated, let 

 us say, as 15-78 or 15-80, still in the earlier stages of the calculation 

 when 2 to 5 c.c. of urine are analysed the results to the third or even 

 to the fourth place are of value so that accumulated rejection errors 

 may be avoided. 



In rejecting figures the rule is to increase by one the last figure 

 retained when the rejected figure is 5 or greater ; otherwise leave 

 it unchanged. Thus 5437 would become 544, but 5434 would 

 remain as 543. 



CHAPTER II 

 THE PROTEINS 



The proteins are a group of substances of first-rate importance 

 to the organism as they are essential constituents in every cell and 

 as foodstuffs they are the main source of nitrogen for the building 

 of new tissue. Most of the members of the group are amorphous 

 substances of high molecular weight. The molecule is made up of 

 the elements carbon, hydrogen, nitrogen, oxygen and sulphur. 

 The percentage amount of these various elements differs in different 

 proteins. 



