What is your favourite experiment? This is a question that is bound to come up in conversations of scientists, class rooms or retreats. It is sort of like: what’s your favourite novel or your favourite painter. It is always difficult to answer because one is bound to be wrong with what it is said on the spur of  the moment. Whatever one  says –and you will know if you have been here- you will change your mind later, because what you have said is what you remembered. Given time you are likely to come up with a list of experiments (or novels, or painters or pieces of music) which would be difficult to tease apart. In the end, logic and emotion will collude to choose a favourite. So, the other day I asked myself: what is my favourite experiment?
As a biologist there is no shortage to choose from. Some of the best and most popular ones come to mind. Avery, MacLeod and McCarty showing that DNA contains the hereditary material, Pasteur’s removing once and for all the notion of spontaneous generation. Of course, Meselson and Stahl’s beautiful proof of semiconservative replication. There is not much to match any of these in the last twenty years, largely because in Biology we have substituted Science for data collecting and gene (I mean bean) counting (something the big journals love). In the physical sciences there are many exceptionally beautiful experiments: the double prism experiment of Newton, the measurement of the bending of light by Eddington, the weighing of Oxygen by Lavoisier……. the list could be long; all tributes to the ingenuity and beauty of the human mind.
As a developmental biologist I have always been seduced by the experiments of Driesch in which he separated the two blastomeres of a sea urchin embryo only to find out that they would form two embryos, rather than two half embryos. And of course, the epic cloning experimentss of John Gurdon, which have an interesting history. These experiments are characterized by their conceptual simplicity but technical challenge to answer an important question.
But when I think about it, my favourite experiment is one that I rarely hear mentioned in this light. It is simple, even boring, but somehow ever since I heard about  it, has captured my imagination. It was performed by R Boyle in 1662 and, as I recently discovered, had the assistance of R Hooke in its design. Boyle had been interested in what he called “the spring in the air’ which led him to what we know as Boyle’s law, namely that Pressure is the inverse of Volume in a gas. While on this subject he had the idea that sound required the deformation of air i.e. that it was a form of pressure in the air. To prove this he prepared a remarkable contraption which allowed a bell to be placed inside a container from which the air could be removed –at the time methods to create vacuum efficiently had been discovered-. Now, he and his friend Hooke figured out a way to manipulate the bell inside the container as the air was being extracted .And herein the beauty of the art. The bell is moved before pumping out the air and it rings, then as the air is drawn out, the sound is dampened until, vacuum created, bell dangled, no sound! QED beautiful, simple, impactful. I can imagine the audience, dumbfounded.
No rational for this choice, many of the others (and many more that you can think of) will do just as well. I guess, all down to the fact that one of the beauties of Science is the sense of awe and wonder and this experiment has a good dose of both. It is also that thinking about experiments like Boyle’s and the others should serve as an inspiration and push us to think about good questions and good experiments to reveal the inner workings of Nature. This is the way it used to be and where we need to return.
I always liked those experiments but loved the simplicity of this one: Jan Baptist van Helmont (1580-1644) measured the weight of a young willow tree and planted it in a pot, which was also measured. Watered for five years. Then measured the difference in weight in the tree and the pot (with its soil) and concluded that the growth of the plant was based on the absorption of water, not the incorporation of soil material. Of course he did not know about CO2 fixation.