65^ 



NA TURE 



[November i i, 1922 



pure iron and lead showed that the effect of strain 

 is a minor and ephemeral factor in corrosion in neutral 

 solutions ; a trace of impurity appears to assist 

 local corrosion, but the amount of corrosion is not 

 proportional to the amount of impurity. The effect 

 of a trace of impurity is probably a trigger action. 

 The main function of oxygen in corrosion is not th.it oJ 

 a depolariser, but rather to oxidise the metal directly, 

 and also in some cases the products of corrosion. 



Two chief types oi corrosion arc distinguished: 

 (a) The general type, usually characteristic of acid 

 corrosion ; and (b) the local type, usually character- 

 istic of corrosion in water and salt solutions. The 

 latter is generally characterised by the formation of 

 an adherent scale on the metal, which may contain 

 colloid. The significance of colloids in corrosion 

 appears to be as follows : A metal immersed in water 

 sends positively charged metal ions into the liquid, 

 and becomes itself negatively charged. With com- 

 mercial metals, the metal also becomes superficially 

 oxidised if dissolved oxygen is present. The 

 hydroxide produced can take up the ions given oft 

 by tin- metal, and thereby becomes a positively 



charged colloid. Some of this will diffuse away, 

 permitting further reaction between oxygen and the 

 metal surface. Oxidation stops until this hydroxide 

 can pass into the colloidal state by acquiring positively 

 charged metal ions. This, in general, does no1 take 

 place till the colloid initially formed has diffused 

 into the presence of electrolyte, when it is pre- 

 cipitated by the anion of the dissolved salt, the 

 cation neutralising the charge on the metal corre- 

 sponding to that on the colloid. Then the metal 

 can send more ions into solution, and the uncharged 

 hydroxide can acquire a charge. If the colloid 

 produced can diffuse away, the process can continue 

 and corrosion develop. If the colloid precipitates 

 directly on the corroding surface it will, in general, 

 adhere and stop corrosion. In the case of a corrosion 

 pit, it is only when the colloid diffuses through an 

 aperture m the gel-deposits at the mouth of the pit 

 that it meets electrolyte and is then precipitated. 

 Such precipitation merely thickens the external 

 gel-deposits. The latter protect the metal surround- 

 ing the pit, and emphasise the local nature of the 

 corrosion. 



Vitamins. 



T"HE Sections of Physiology and Agriculture of 

 -1 the British Association held a joint discussion 

 on vitamins at Hull on Friday, September 8. 



Prof. J. C. Drummond spoke of the great strides 

 that have been made since the discovery of the 

 vitamins by Hopkins in 1912. Both the existence 

 and the indispensability of these substances are now 

 generally accepted. The far-reaching importance 

 of the qualitative composition of the diet of man 

 and animals is being gradually appreciated, and the 

 signifii ance of those factors which exist in extremely 

 minute amounts recognised. Three substances of 

 the so-called vitamin class have been differentiated 

 with certainty, and it is possible that more exist. 

 They do not appear to be of one chemical type, and 

 the only ground for grouping them together is that 

 they occur, and are effective, in very small amounts. 

 Parallel examples from the inorganic food con- 

 stituents are known, such as the value of minute 

 doses of iodides in the treatment and prevention of 

 foetal athyrosis in swine. 



The green tissues of plants would seem to be the 

 chief site of vitamin synthesis, although lower forms 

 of plant life devoid of photocatalytic pigments can 

 apparently produce the vitamin B. Plant tissues 

 undoubtedly form the direct or indirect source of the 

 vitamin supply of animals, but we are entirely ignorant 

 as to the role of the vitamins in the plant itself. 



Storage of the vitamin A may take place in the 

 tissues, liver, and body fat of animals, and may 

 serve as a reserve from which are drawn supplies to 

 maintain the vitamin concentration of milk if the 

 diet during the lactation period should be deficient. 



In collaboration with Dr. Zilva a prolonged in* 

 vestigation of the origin of the large stores of vitamin 

 A in cod-liver oils has recently been made. It has 

 been ascertained that the marine diatoms synthesise 

 the vitamin, and that it is transferred to the tissues 

 of minute animals (plankton) which thrive on the 

 unicellular plants. These in turn form the food 

 supply of larger species, particularly * small fish, 

 which in their turn are devoured by the larger fish, 

 such as the cod. Through all these stages there is 

 apparently a transference of the vitamin, ending 

 finally in the storage in the liver of the cod. The 

 modern methods of manufacture of cod - liver oil 

 do not appreciably lower the vitamin value, but 



NO. 2767, VOL. I IO] 



there are wide variations in the value of different 

 samples which are probably connected with the 

 seasonal changes in the feeding habits or physio- 

 logical condition of the fish. Considerable work has 

 been done on the chemical nature of the vitamin A, 

 but an isolation has not yet been made. It is very 

 stable, except to oxidative changes, and passes into 

 the unsaponifiable fraction of the oil. Cholesterol, 

 pigments, and other fractions of this fraction may be 

 removed without loss of potency. 



Capt. J. Golding gave a number of illustrations of 

 the vahie of the application of vitamin theories in 

 practical pig-feeding. Frequently the usual type of 

 pig diet is deficient in vitamins, particularly vitamin 

 A, and the beneficial influence of cod-liver oil or of 

 feeding on pasture or lucerne in such cases is remark- 

 able. In the compounding of rations care should be 

 taken to ensure an adequate supply of food-stuffs 

 rich in vitamins, otherwise there is danger of sub- 

 normal growth, impaired resistance to infections, 

 and disturbances of the power to produce and rear 

 normal young. The majority of the cereal products 

 are deficient in vitamin A, and the amount in the 

 diet is not raised much by the use of separated milk. 

 Such diets can be supplemented by small additions 

 of cod-liver oil, 1-2 oz. daily for full-grown pigs, or 

 by access to pasture. Cod-liver oil is also valuable 

 in maintaining the vitamin value of the milk yielded 

 by cows on winter rations in stall, which otherwise 

 tends to fall. The administration of cod-liver oil, 

 if of good quality, does not produce flavour or taint 

 in pigs or milk and butter. 



Dr. Atherton Seidell (New York) described his 

 attempts at the separation of the vitamin B from 

 yeast by chemical methods. By adsorption of the 

 vitamin from yeast extracts on to fuller's earth, and 

 extraction of the activated solid with alkalies under 

 suitable conditions, considerable concentration of the 

 active substance could be effected. The resulting 

 extract when fractionated by precipitation with silver 

 salts gave active fractions, but these have not yet 

 yielded a pure substance. 



Prof. W. I). Halliburton referred to the need for 

 caution that enthusiasm for a new word such as 

 vitamin did not overwhelm the importance of other 

 dietary units. There must not be a loss of per- 

 spective in viewing the function of these newly 



