"Van Helmont of ... was formed in the school of Alchemy, and his mind was tinctured with its prejudices: but his views concerning nature and the elements were distinguished by much more philosophical acuteness, and more sagacity, than those of any former writer. He is the first person who seems to have had any idea respecting elastic fluids, different from the air of the atmosphere; and he has distinctly mentioned three of these substances, to which he applied the term es: namely aqueous gas or steam, unctuous or inflammable gas, and gas from wood or carbonic acid gas. Van Helmont developed some accurate views respecting the permanent elasticity of air, and the operation of heat upon it; and a sketch of a curious instrument very similar to the differential thermometer, is to be found in his works."