Although his background was in alchemy, Robert Boyle is thought to be one of the founders of modern chemistry, if not the first modern chemist. In addition to chemistry, he dabbled in physics, philosophy, and inventions. Many of his theories were published in "The Sceptical Chymist," which was published in 1661. These relate from a wide range of subjects that range from combustion and respiration, to how colors are formed and the chemical breakdown of solid matter. Boyle was born in Ireland yet traveled around Europe for his academic training, including a winter spent studying in Florence with Galileo. He then returned to London to carry out much of his professional career.
The most important discovery that Robert Boyle remains well known for is Boyle's Law. This discusses the inversely proportional relationship between the pressure and volume of a gas, as long as the gas is kept at a stable temperature. The basics of this law had already been hypothesized by Richard Towneley and Henry Power, and then were proven through scientific method by Boyle and his assistant, Robert Hooke. In basic terms, if the pressure goes up, then the volume will go down. In the 1600's, there was little capacity for extreme temperatures so this mainly relates to stable temperature.
Another area in which Robert Boyle was quite well known for his contributions in was the elements. He believed in elements as existing to be indecomposable particles of larger material forms or bodies, and distinguished in between compounds and mixtures. He and his assistants worked hard on techniques to detect their various different ingredients, which was then called "analysis" in his workshop. Some of his other contributions in the world of physics include his discover of the role that air takes in the propagation of sound, the different effects of gravity, the compounds of crystals, and the fundamentals of electricity.
For his contributions to the scientific method and research into such phenomena as the breakdown of elemental particles and forms, Robert Boyle has earned his title as the first modern chemist. He did a great deal of work and his findings are still read in their published forms to this day. Although he is quite well known for his work in the physics world, it was his ideas and experiments about chemistry, and his passion for the field, that have made him stand out as one of the fathers of chemistry.