Newton demonstrated that white light is composed of rays of various colors, and that the color reflected by any object is due to the ability of the object to reflect certain rays while absorbing the rest. The Dutch physicist, Huygens, championed the undulatory or wave theory of light. Refraction was explained by both Newton and Huygens, and the latter, while studying the double refraction of crystals of Iceland spar, discovered the phenomena of polarization. Boyle's chemical discoveries led to much research in chemistry. Black, Bergman and Van Helmont investigated the properties of carbonic acid gas. Joseph Black treated limestone with acid and collected the gas evolved in a Hales pneumatic trough. He weighed the gas and the remainder of the limestone, finding that what the limestone lost was equivalent to the weight of the gas. He then reversed the process and succeeded in making chalk from a solution of lime. This simple experiment paved the way for chemical analysis and syntheses which have added profoundly to our knowledge of the composition of matter. Bergman tested Black's gas with litmus and found it gave an acid reaction and in 1779 Lavoisier demonstrated that it consisted of carbon and oxygen. Priestley and Cavendish, both English chemists, then took up this study. Cavendish treated iron, tin, zinc, and other metals with sulphuric acid and discovered a new gas which he termed hydrogen. Rutherford discovered nitrogen in 1772 and Priestley isolated nitric oxide, and in 1774 discovered oxygen. In the course of his experiment Priestley also discovered ammonia, sulphur dioxide and other chemicals. His greatest achievements, however, were the isolation and recognition of oxygen, and the discovery of the composition of water. Following up these discoveries, he noted that the air is not a simple elementary substance, but a mixture of nitrogen and oxygen with several impure gases. The work of this great chemist became as fruitful in the chemical field as that of Newton in physics, astronomy, and mathematics. Carl Wilhelm Scheele, a Swede, carried out many experiments which resulted in the discovery of tartaric acid, the decomposition of silver chloride by light, magnesium nitrate, magnesia, microcosmic salt, and sulphureted hydrogen, chlorine, hydrofluoric, and other inorganic acids. He also discovered the following organic acids: lactic, gallic, pyrogallic, oxalic, citric, malic, mucic and uric. He isolated glycerin and sugar of milk and determined the nature of hydrocyanic acid, borax, plumbago, Prussian blue, and other chemicals. He invented many new chemical and laboratory processes. Scheele was an apothecary's assistant and lived in poverty. But although his experiments were conducted under disadvantageous circumstances his discoveries ranked him as the greatest chemist of his time and one of the greatest chemical experimenters of all time.