It follows that, if there were no percipient being to receive these sensations, there would be no heat or cold, no taste, no smell, no sound, and no colour. Aware of this difference between our sensations in certain cases and the causes of these sensations, Locke has divided the qualities of substances in the material universe into primary and secondary, the sensations we receive of the primary representing the actual qualities of material substances, but the sensations we receive of what he calls the secondary having no proper resemblance to the causes that produce them. Now, if we proceed in the spirit of severe analysis to examine the primary qualities of matter, we shall not perhaps find so marked a distinction between those and the secondary, as the statement of Locke would have led us to imagine. The Optics of Newton were published fourteen years later than Locke's Essay concerning Human Understanding. In endeavouring to account for the uninterrupted transmission of rays of light through transparent substances, however hard they may be found to be, Newton has these observations. "Bodies are much more rare and porous, than is commonly believed. Water is nineteen times lighter, and by consequence nineteen times rarer, than gold; and gold is so rare, as very readily, and without the least opposition, to transmit the magnetic effluvia, and easily to admit quicksilver into its pores, and to let water pass through it. From all which we may conclude, that gold has more pores than solid parts, and by consequence that water has above forty times more pores than parts. And he that shall find out an hypothesis, by which water may be so rare, and yet not capable of compression by force, may doubtless, by the same hypothesis, make gold, and water, and all other bodies, as much rarer as he pleases, so that light may find a ready passage through transparent substances(43)." (43) Newton, Optics, Book II, Part III, Prop. viii.