From the Greek, eudios ('clear weather') and 'meter', the eudiometer is an instrument for testing the purity of air, or rather the quantity of oxygen it contains. It is now chiefly employed in the analsysis of gases. Ruskin does not mention the instrument apart from here, and there is no clear evidence that he used one. By citing it, Ruskin was demonstrating his scientific knowledge - whilst voicing his characteristic warning that science was flawed without art.