4 oo SCIENCE PROGRESS 



the soil, so that more oxygen must diffuse towards the worms 

 from the air above to take its place. Thus a constant inter- 

 change of gases is maintained. 



Assuming with Darwin that there are some 27,000 worms 

 existing per acre of soil, it should be possible to calculate the 

 rate of interchange of gases thus set up, provided we know the 

 relative distribution of the different species present. Unfortun- 

 ately this is not known but taking the mean of the large and 

 small species the average worm may be assumed to weigh o"6 

 gram and to have a carbon dioxide factor of 1/200. Each worm 

 will then exhale o - oo3 gram of carbon dioxide in twenty-four 

 hours ; 27,000 worms will exhale 81 grams or approximately 40 

 litres of the gas. In the course of a year this would amount to 

 29/6 kilograms or 15,000 litres of carbon dioxide per acre. Since 

 for every litre of gas evolved an equal volume of oxygen must 

 find its way into the soil from the air, it is clear that for the 

 whole of Great Britain, which contains many millions of acres, 

 the total gaseous interchange due to this cause alone must be 

 very considerable. 



Incidentally it is interesting to note that the carbon dioxide 

 exhaled per acre during a whole }^ear is sufficient under favour- 

 able conditions to effect the solution of 377 kilograms (83 lb.) 

 of metallic iron. This amount is certainly not negligible, parti- 

 cularly when we remember that during corrosion, when the 

 dissolved metal is thrown out of solution as rust, the acid is 

 liberated and is thus rendered free to attack more iron. Further- 

 more, Darwin states that some worms of his were kept in test 

 pots filled with sand of a red colour, due to the presence of 

 anhydrous ferric oxide — a slightly soluble body and one from 

 which we can imagine plants are unable to assimilate the iron 

 directly. It was observed, however, that the burrows made by 

 the worms were lined with their castings, as usual, composed of 

 colourless sand and intestinal secretions. Examination under 

 the microscope proved that the ferric oxide had been completely 

 dissolved out of the sand by the digestive action of the worms, 

 although it was of such an insoluble nature that the usual 

 mineral acids diluted according to the British Pharmacopeia 

 produced less effect. It is to be presumed that this iron either 

 entered into the composition of the body of the worm itself or, 

 more likely, was excreted in some soluble form. In either case 

 the iron must eventually be given back to the soil in a more 



