The expression is often met with in cookery books "Thicken with a little butter and flour," or "Thicken with a little arrowroot;" but this expression requires some little explanation for novices. Besides, there is a great art in thickening, and many nice dishes have been spoilt owing to cooks being ignorant of the proper method. The commonest ways of thickening are with flour, corn-flour or arrowroot, butter and flour, and eggs. Perhaps the most useful method of thickening is with butter and flour mixed. But I must here impress on you the importance of first, so to speak, "cooking" this butter and flour. Suppose you have some thick soup, say, mock-turtle. If you thicken this with butter and flour plain, the soup will have a gruelly taste and a pale brown colour. If you thicken with butter and flour cooked - i.e., with brown thickening - which does not cost one farthing more, you have a rich brown soup worthy of the Freemason's Tavern. Indeed, the difference between good mock-turtle soup and poor gruelly mess is often simply, that one is thickened with butter and flour, and the other with brown thickening, or what the French call " brown roux ": the cost of this being only a few minutes' trouble. I will describe the different ways of thickening under their various headings.