1899] SCIENTIFIC EXPLANATIONS 251 
learning of how many, or rather of how few, different kinds of thread 
it is woven, finding how each separate thread enters into the pattern, 
and seeking from the pattern woven in the past to know the pattern 
yet to come.” ... We have heard from unfriendly critics much in 
regard to the dogmatism of science; it is time rather to speak of its 
modesty. 
An Unsolved Problem. 
In his opening address to the Chemical Section of the British Associa- 
tion, Dr. Horace T. Brown not unnaturally took for his subject the 
fixation of carbon by plants, a problem towards the solution of which 
he has himself made some notable contributions. The address is a 
fine illustration of the true scientific temper, and of the value to 
biologists of co-operation with workers in chemistry and _ physics. 
Definite results are still far to seek, but the address indicates a hopeful 
outlook, and it also impresses us anew with the danger of hard and 
fast statements, and with the incipient character of vegetable physiology. 
The president of Section B began by pointing out that although 
We cease not to impress upon our students that the higher plants 
derive the whole of their carbon from atmospheric sources, the experi- 
mental evidence for this hard and fast statement is very indirect. 
“There can, of course, be no doubt that the primary source of the 
organic carbon of the soil, and of the plants growing on it, is the 
atmosphere; but of late years there has been such an accumulation of 
evidence tending to show that the higher plants are capable of being 
nourished by the direct application of a great variety of ready-formed 
organic compounds, that we are justified in demanding further proof 
that the stores of organic substances in the soil must necessarily be 
oxidised down to the lowest possible point, before their carbon is once 
more in a fit state to be assimilated.” Along with Mr. F. Escombe, 
Dr. Brown has been recently experimenting in order if possible to reach 
some satisfactory answer to this important question. “Up to the 
present time,” he says, “our experiments have not been carried far 
enough to enable us to give a positive answer to the main question, 
but they have already suggested a new method of attack which will 
enable us in the future to determine, with a fair amount of certainty, 
whether any particular plant, growing under perfectly natural conditions, 
derives any appreciable portion of its carbon from any other source than 
the gaseous carbon dioxide of the atmosphere.” 
The address contains a valuable critical account of what has been 
done in the past, and we venture to quote the summing-up. It does 
not sound altogether encouraging, but there is no object in blinking 
the facts. “The brilliant discoveries of recent years on the constitu- 
tion and synthesis of the carbohydrates have not brought us sensibly 
